Fix autocorrection for Cop/LineBreakAfterGuardClauses

What does this MR do and why?

This commit fixes the autocorrection functionality for Cop/LineBreakAfterGuardClauses and improves its spec coverage.

Coverage

Before 99.51%	After 99.83% 🚀
https://gitlab.com/gitlab-org/ruby/gems/gitlab-styles/-/jobs/3601173860	https://gitlab.com/gitlab-org/ruby/gems/gitlab-styles/-/jobs/3601473389

How to verify locally

1. Create foo.rb with

# frozen_string_literal: true
def foo
  return if foo
  bar
end

2. Switch to master and run:

bundle exec rubocop --autocorrect --only Cop/LineBreakAfterGuardClauses foo.rb
Inspecting 1 file
C

Offenses:

foo.rb:3:3: C: Cop/LineBreakAfterGuardClauses: Add a line break after guard clauses
  return if foo
  ^^^^^^^^^^^^^

1 file inspected, 1 offense detected

4. Verify that foo.rb was not corrected ❌

5. Switch to this branch and run:

bundle exec rubocop --autocorrect --only Cop/LineBreakAfterGuardClauses foo.rb
Inspecting 1 file
C

Offenses:

foo.rb:3:3: C: [Corrected] Cop/LineBreakAfterGuardClauses: Add a line break after guard clauses
  return if foo
  ^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

6. Verify that foo.rb got corrected and a newline was added ✅

# frozen_string_literal: true
def foo
  return if foo

  bar
end