Dpkg::Source::Patch: change an error into a warning

* scripts/Dpkg/Source/Patch.pm (analyze): An empty patch
is still valid, instead of failing simply generate a warning.
21 packages in the archive have an empty .diff.gz and
they can't be extracted without this fix.

scripts/Dpkg/Source/Patch.pm

@@ -369,7 +369,7 @@ sub analyze {
     }
     close($diff_handle);
     unless ($diff_count) {
-	error(_g("diff `%s' doesn't contain any patch"), $diff);
+	warning(_g("diff `%s' doesn't contain any patch"), $diff);
     }
     $self->cleanup_after_open();
     $self->{'analysis'}{$destdir}{"dirtocreate"} = \%dirtocreate;