Fix an error for Style/Semicolon

This PR fixes the following error for `Style/Semicolon`
when using a comment containing a semicolon before a block:

```ruby
# ;
foo {
}
```

```console
% bundle exec rubocop --only Style/Semicolon
(snip)

  NoMethodError:
    undefined method `semicolon?' for nil:NilClass
```

No changelog entry because this is a bug by #11465 and not released.

lib/rubocop/cop/style/semicolon.rb

@@ -100,7 +100,7 @@ def exist_semicolon_before_right_curly_brace?(tokens)
         end
 
         def exist_semicolon_after_left_curly_brace?(tokens)
-          tokens[1]&.left_curly_brace? && tokens[2].semicolon?
+          tokens[1]&.left_curly_brace? && tokens[2]&.semicolon?
         end
 
         def register_semicolon(line, column, after_expression, token_before_semicolon = nil)

spec/rubocop/cop/style/semicolon_spec.rb

@@ -142,6 +142,14 @@ module Foo; end
     RUBY
   end
 
+  it 'does not register an offense when using a comment containing a semicolon before a block' do
+    expect_no_offenses(<<~RUBY)
+      # ;
+      foo {
+      }
+    RUBY
+  end
+
   it 'registers an offense for range (`1..42`) with semicolon' do
     expect_offense(<<~RUBY)
       1..42;