Access keys via get, to prevent KeyErrors

Fixes #19

funkwhale_cli/cli/tracks.py

@@ -79,12 +79,12 @@ async def track_download(
             logs.logger.info("Downloading from {}".format(download_url))
 
             filename_params = utils.flatten(track_data)
-            filename_params["album"] = filename_params["album_title"]
-            filename_params["artist"] = filename_params["artist_name"]
+            filename_params["album"] = filename_params.get("album_title", None)
+            filename_params["artist"] = filename_params.get("artist_name", None)
             filename_params["extension"] = format
             filename_params["year"] = (
-                filename_params["album_release_date"][:4]
-                if filename_params["album_release_date"]
+                filename_params.get("album_release_date", None)[:4]
+                if filename_params.get("album_release_date", None)
                 else None
             )
             filename_params = {