Freeze string for default content for tag_strings, which later goes o…

…n to create new content. This should also be inline with content being passed should not be mutable

actionview/lib/action_view/helpers/tag_helper.rb

@@ -41,7 +41,7 @@ def tag_string(name, content = nil, escape_attributes: true, **options, &block)
           if VOID_ELEMENTS.include?(name) && content.nil?
             "<#{name.to_s.dasherize}#{tag_options(options, escape_attributes)}>".html_safe
           else
-            content_tag_string(name.to_s.dasherize, content || '', options, escape_attributes)
+            content_tag_string(name.to_s.dasherize, content || ''.freeze, options, escape_attributes)
           end
         end