Add expect_offense mark for offense on empty line

[biinari]

Part of #8003

Use ^{} to denote an offense at an empty line in expect_offense. This format was suggested by @marcandre in Issue 8003 comment. I think it has the advantage of familiarity with existing ^ and ^{name} syntax.

Example:

expect_offense(<<~RUBY)

  ^{} Missing frozen string literal comment.
  puts 1
RUBY

I have enabled pending frozen string literal specs as coverage for this change.

Also corrected which line the offense is added to in the frozen string literal
specs (always the first line). As the first line is non-empty in some of these
cases, a normal caret ^ is used to mark the offense.

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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[bbatsov]

I like this! Thanks!