Fix header Content-Type: #<Mime::NullType:...> in localized template

This PR is the backport of #eb448efi.
It fixes #13064 regression bug introduced by the #8085

Now in _process_format when the format is a Mime::NullType nothing is written in self.content_type.
In this way the method Response#assign_default_content_type_and_charset can
write the the default mime_type.