Avoid crash in Style/HashSyntax cop with an empty hash

[pocke]

Problem

Style/HashSyntax cop crashes with an empty hash when EnforcedStyle is no_mixed_keys.

 # .rubocop.yml
Style/HashSyntax:
  EnforcedStyle: no_mixed_keys
  Enabled: true

 # test.rb
{}

$ rubocop --only Style/HashSyntax --debug
An error occurred while Style/HashSyntax cop was inspecting /tmp/tmp.iBl3LTfZjB/test.rb.

1 error occurred:
An error occurred while Style/HashSyntax cop was inspecting /tmp/tmp.iBl3LTfZjB/test.rb.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
Mention the following information in the issue report:
0.46.0 (using Parser 2.3.3.1, running on ruby 2.3.3 x86_64-linux)
For /tmp/tmp.iBl3LTfZjB: configuration from /tmp/tmp.iBl3LTfZjB/.rubocop.yml
Default configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/config/default.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/config/enabled.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/config/disabled.yml
Inspecting 1 file
Scanning /tmp/tmp.iBl3LTfZjB/test.rb
undefined method `loc' for nil:NilClass
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/style/hash_syntax.rb:124:in `no_mixed_keys_check'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/style/hash_syntax.rb:88:in `on_hash'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:41:in `block (2 levels) in on_hash'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:96:in `with_cop_error_handling'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:40:in `block in on_hash'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:39:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:39:in `on_hash'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/ast_node/traversal.rb:12:in `walk'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:58:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:120:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:108:in `offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:51:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:243:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:190:in `block in do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:222:in `block in iterate_until_no_changes'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:215:in `loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:215:in `iterate_until_no_changes'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:186:in `do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:101:in `block in file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:111:in `file_offense_cache'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:99:in `file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:90:in `process_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:68:in `block in each_inspected_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `reduce'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `each_inspected_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:57:in `inspect_files'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:36:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cli.rb:72:in `execute_runner'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cli.rb:27:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/bin/rubocop:13:in `block in <top (required)>'
/usr/lib/ruby/2.3.0/benchmark.rb:308:in `realtime'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/bin/rubocop:12:in `<top (required)>'
/home/pocke/.gem/ruby/2.3.0/bin//rubocop:22:in `load'
/home/pocke/.gem/ruby/2.3.0/bin//rubocop:22:in `<main>'
.

1 file inspected, no offenses detected
Finished in 0.14013576700381236 seconds

I fixed the problem.

Note

I've forgotten to run rake generate_cops_documentation in #3765 .

---

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
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• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).