Add autocorrect for Lint/EmptyEnsure cop

[pocke]

$ cat test.rb
begin
  something
ensure
end

$ rubocop -a
Inspecting 1 file
W

Offenses:

test.rb:3:1: W: [Corrected] Empty ensure block detected.
ensure
^^^^^^
test.rb:3:1: C: [Corrected] Extra empty line detected at begin body end.

1 file inspected, 2 offenses detected, 2 offenses corrected

$ cat test.rb
begin
  something
end

---

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.
• 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).

[bbatsov]

This auto-correct should be disabled by default. Lint cops never have clear-cut auto-correction - e.g. you might have accidentally forgotten the body of an ensure and deleting this is going to cover that up.

[pocke]

This auto-correct should be disabled by default. Lint cops never have clear-cut auto-correction - e.g. you might have accidentally forgotten the body of an ensure and deleting this is going to cover that up.

I understand. I've changed the default.