Add expect_offense mark for offense on empty line

Part of rubocop#8003

Use `^{}` to denote an offense at an empty line in `expect_offense`.

Example:

```ruby
expect_offense(<<~RUBY)

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

Enable pending frozen string literal specs

Correct which line the offense is added to (always the first line).

As the first line is non-empty in these cases, use a normal caret `^` to
mark the offense.

lib/rubocop/rspec/expect_offense.rb

@@ -89,6 +89,16 @@ module RSpec
     #       end
     #     RUBY
     #   end
+    #
+    # If you need to specify an offense on a blank line, use the empty `^{}` marker:
+    #
+    # @example `^{}` empty line offense
+    #
+    #   expect_offense(<<~RUBY)
+    #
+    #     ^{} Missing frozen string literal comment.
+    #     puts 1
+    #   RUBY
     module ExpectOffense
       def format_offense(source, **replacements)
         replacements.each do |keyword, value|
@@ -177,7 +187,7 @@ def expect_no_offenses(source, file = nil)
 
       # Parsed representation of code annotated with the `^^^ Message` style
       class AnnotatedSource
-        ANNOTATION_PATTERN = /\A\s*\^+ /.freeze
+        ANNOTATION_PATTERN = /\A\s*(\^+|\^{}) /.freeze
 
         # @param annotated_source [String] string passed to the matchers
         #
@@ -261,6 +271,7 @@ def with_offense_annotations(offenses)
             offenses.map do |offense|
               indent     = ' ' * offense.column
               carets     = '^' * offense.column_length
+              carets     = '^{}' if offense.column_length.zero?
 
               [offense.line, "#{indent}#{carets} #{offense.message}\n"]
             end

spec/rubocop/cop/style/frozen_string_literal_comment_spec.rb

@@ -273,12 +273,9 @@
     end
 
     it 'registers an offense for an extra first empty line' do
-      pending 'There is a flaw that skips adding caret symbol in this case, ' \
-              'making it impossible to use `expect_offense` matcher'
-
       expect_offense(<<~RUBY)
 
-        ^ Missing magic comment `# frozen_string_literal: true`.
+        ^{} Missing frozen string literal comment.
         puts 1
       RUBY
 
@@ -580,12 +577,9 @@
     end
 
     it 'registers an offense for an extra first empty line' do
-      pending 'There is a flaw that skips adding caret symbol in this case, ' \
-              'making it impossible to use `expect_offense` matcher'
-
       expect_offense(<<~RUBY)
 
-        ^ Missing magic comment `# frozen_string_literal: true`.
+        ^{} Missing magic comment `# frozen_string_literal: true`.
         puts 1
       RUBY
 
@@ -854,14 +848,11 @@
 
     it 'registers an offense for not having a frozen string literal comment ' \
        'under a shebang, an encoding comment, and extra space' do
-      pending 'There is a flaw that skips adding caret symbol in this case, ' \
-              'making it impossible to use `expect_offense` matcher'
-
       expect_offense(<<~RUBY)
         #!/usr/bin/env ruby
+        ^ Missing magic comment `# frozen_string_literal: true`.
         # encoding: utf-8
 
-        ^ Missing magic comment `# frozen_string_literal: true`.
         puts 1
       RUBY
 
@@ -927,13 +918,10 @@
 
     it 'registers an offense for not having a frozen string literal comment ' \
        'under an encoding comment and extra space' do
-      pending 'There is a flaw that skips adding caret symbol in this case, ' \
-              'making it impossible to use `expect_offense` matcher'
-
       expect_offense(<<~RUBY)
         # encoding: utf-8
-
         ^ Missing magic comment `# frozen_string_literal: true`.
+
         puts 1
       RUBY