patches.cgi: Do not mark 'Forwarded: yes' as invalid
Per DEP-3, the "Forwarded" field does not have to be a url, it may contain "yes".
Previously UDD was marking patches as having invalid metadata is no url was given, but sometimes there is no url as upstream as no bug tracker, only an email address.
This commit modifies rimporters/patches.rb to not mark such patches as invalid, and web/patches.cgi to only create a if a url is present.
Closes: #104343