Fix RuboCop crash with empty brackets in Style/Next cop

[pocke]

Problem

RuboCop crashes with empty brackets on Style/Next cop.

Reproduce

test.rb

loop do
  ()
end

$ rubocop -v
0.42.0
$ rubocop -d test.rb --cache false
For /home/pocke/ghq/github.com/bbatsov/rubocop: configuration from /home/pocke/ghq/github.com/bbatsov/rubocop/.rubocop.yml
Inheriting configuration from /home/pocke/ghq/github.com/bbatsov/rubocop/.rubocop_todo.yml
Default configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/config/default.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/config/enabled.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/config/disabled.yml
Inspecting 1 file
Scanning /home/pocke/ghq/github.com/bbatsov/rubocop/test.rb
An error occurred while Style/Next cop was inspecting /home/pocke/ghq/github.com/bbatsov/rubocop/test.rb.
undefined method `if_type?' for nil:NilClass
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/style/next.rb:92:in `if_without_else?'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/style/next.rb:84:in `simple_if_without_break?'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/style/next.rb:80:in `ends_with_condition?'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/style/next.rb:44:in `on_block'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/commissioner.rb:42:in `block (2 levels) in on_block'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/commissioner.rb:97:in `with_cop_error_handling'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/commissioner.rb:41:in `block in on_block'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/commissioner.rb:40:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/commissioner.rb:40:in `on_block'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/ast_node/traversal.rb:13:in `walk'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/commissioner.rb:59:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/team.rb:121:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/team.rb:109:in `offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cop/team.rb:52:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:223:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:193:in `block in do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:183:in `loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:183:in `do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:93:in `block in file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:103:in `file_offense_cache'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:91:in `file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:82:in `process_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:59:in `block in inspect_files'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:57:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:57:in `inspect_files'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/runner.rb:35:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cli.rb:72:in `execute_runner'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/lib/rubocop/cli.rb:28:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.42.0/bin/rubocop:14: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-0.42.0/bin/rubocop:13:in `<top (required)>'
/home/pocke/.gem/ruby/2.3.0/bin//rubocop:23:in `load'
/home/pocke/.gem/ruby/2.3.0/bin//rubocop:23:in `<main>'
C

Offenses:

test.rb:1:1: C: Style/Encoding: Missing utf-8 encoding comment.
loop do
^^^^^^^
test.rb:1:1: C: Style/FrozenStringLiteralComment: Missing frozen string literal comment.
loop do
^

1 file inspected, 2 offenses detected

1 error occurred:
An error occurred while Style/Next cop was inspecting /home/pocke/ghq/github.com/bbatsov/rubocop/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:
0.42.0 (using Parser 2.3.1.2, running on ruby 2.3.1 x86_64-linux)
Finished in 0.06772538200311828 seconds

---

Before submitting the PR make sure the following are checked:

• 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).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.