set pseudo extension for Metadata messages (#865)

This prevents pathfmt.filename from potentially being empty.

gallery_dl/extractor/patreon.py

@@ -38,8 +38,7 @@ class PatreonExtractor(Extractor):
             hashes = set()
 
             yield Message.Directory, post
-            yield Message.Metadata, text.nameext_from_url(
+            yield Message.Metadata, post
-                post["creator"].get("image_url", ""), post)
 
             for kind, url, name in itertools.chain(
                 self._postfile(post),

gallery_dl/job.py

@@ -281,6 +281,7 @@ class DownloadJob(Job):
         postprocessors = self.postprocessors
 
         if postprocessors:
+            kwdict["extension"] = "metadata"
             pathfmt = self.pathfmt
             pathfmt.set_filename(kwdict)
             for pp in postprocessors: