www: gzip_filter: avoid errors after ->write failure
->zflush must return a string to its caller, not undef.
Additionally, {http_out} may be deleted on ->write if ->close
recurses.
This should fix the following errors:
Use of uninitialized value $_[1] in string eq at PublicInbox/HTTP.pm line 211.
E: Can't call method "close" on an undefined value at GzipFilter.pm line 167.