Make the frozen_string Danger check more resilient

It's possible for a Ruby file to be empty and thus `gets` returns `nil`.

Also indents the Markdown since we're using `<<~` and removes an
unneeded variable.

danger/frozen_string/Dangerfile

@@ -6,22 +6,21 @@ MAGIC_COMMENT = "# frozen_string_literal: true"
 def get_files_with_no_magic_comment(files)
   files.select do |file|
     file.end_with?(FILE_EXTENSION) &&
-      !File.open(file, &:gets).start_with?(MAGIC_COMMENT)
+      !File.open(file, &:gets)&.start_with?(MAGIC_COMMENT)
   end
 end
 
-files_to_check = git.added_files
-files_to_fix = get_files_with_no_magic_comment(files_to_check)
+files_to_fix = get_files_with_no_magic_comment(git.added_files)
 
 if files_to_fix.any?
   warn 'This merge request adds files that do not enforce frozen string literal. ' \
     'See https://gitlab.com/gitlab-org/gitlab-ce/issues/47424 for more information.'
 
   markdown(<<~MARKDOWN)
-## Enable Frozen String Literal
+    ## Enable Frozen String Literal
 
-The following files should have `#{MAGIC_COMMENT}` in the first line:
+    The following files should have `#{MAGIC_COMMENT}` on the first line:
 
-* #{files_to_fix.map { |path| "`#{path}`" }.join("\n* ")}
+    * #{files_to_fix.map { |path| "`#{path}`" }.join("\n* ")}
   MARKDOWN
 end