Add autocorrection to Lint/EmptyConditionalBody

[dvandersluis]

I noticed in #10858 (comment) that Lint/EmptyConditionalBody does not have autocorrection, so I added it. It does the following:

• Removes the empty branch, including any comments if AllowComments is false.
• Handles subsequent branches, including changing elsif to if and else to if/unless appropriately, if the topmost branch is the one being removed.
• Can handle the if statement being preceded by another node.

I believe I've covered the edge cases here but if there's one you can think of let me know and I'll add it in. I also was unsure if we should mark this as SafeAutoCorrect: false but am happy to do so.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[BrianHawley]

@dvandersluis this correction doesn't have the same semantics. To get the same meaning, it should be:

if !condition && other_condition
  do_something
elsif !condition && another_condition
  do_something_else
end

I don't know if this autocorrection was considered safe before, but it definitely isn't safe or correct now.

[dvandersluis]

Yeah I noticed this after it was merged, and it's been changed to SafeAutoCorrect: false. I haven't had a chance to look at fixing the autocorrection yet so feel free to create a PR!

[BrianHawley]

Same, this should be:

x = if !foo && bar
  5
end