Fix an incorrect auto-correct for Layout/DotPosition

[koic]

This PR fixes the following incorrect auto-correct for Layout/DotPosition when using only dot line.

% cat example.rb
foo
  .bar
  .
  baz

% rubocop --only Layout/DotPosition -a
(snip)

Inspecting 1 file
C

Offenses:

example.rb:3:3: C: [Corrected] Layout/DotPosition: Place the . on the
next line, together with the method name.
  .
  ^

1 file inspected, 1 offense detected, 1 offense corrected

Before

% cat example.rb
foo
  .bar

  .baz

% ruby -c example.rb
example.rb:4: syntax error, unexpected '.', expecting end-of-input
  .baz

After

% cat example.rb
foo
  .bar
  .baz

% ruby -c example.rb
Syntax OK

---

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.

[bbatsov]

Thanks!