Make local actor icon optional

If a remote actor has no icon, we show our local default icon.

If we have no icon, we should allow remote instances to show their
default icon, instead of sending ours.

app/main.py

@@ -1431,7 +1431,7 @@ async def json_feed(
                 ],
             }
         )
-    return {
+    result = {
         "version": "https://jsonfeed.org/version/1",
         "title": f"{LOCAL_ACTOR.display_name}'s microblog'",
         "home_page_url": LOCAL_ACTOR.url,
@@ -1439,10 +1439,12 @@ async def json_feed(
         "author": {
             "name": LOCAL_ACTOR.display_name,
             "url": LOCAL_ACTOR.url,
-            "avatar": LOCAL_ACTOR.icon_url,
         },
         "items": data,
     }
+    if LOCAL_ACTOR.icon_url:
+        result["author"]["avatar"] = LOCAL_ACTOR.icon_url
+    return result
 
 
 async def _gen_rss_feed(
@@ -1454,7 +1456,8 @@ async def _gen_rss_feed(
     fg.description(f"{LOCAL_ACTOR.display_name}'s microblog")
     fg.author({"name": LOCAL_ACTOR.display_name})
     fg.link(href=LOCAL_ACTOR.url, rel="alternate")
-    fg.logo(LOCAL_ACTOR.icon_url)
+    if LOCAL_ACTOR.icon_url:
+        fg.logo(LOCAL_ACTOR.icon_url)
     fg.language("en")
 
     outbox_objects = await _get_outbox_for_feed(db_session)