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/activitypub.py

@@ -135,11 +135,6 @@ ME = {
     "url": config.ID + "/",  # XXX: the path is important for Mastodon compat
     "manuallyApprovesFollowers": config.CONFIG.manually_approves_followers,
     "attachment": _LOCAL_ACTOR_METADATA,
-    "icon": {
-        "mediaType": mimetypes.guess_type(config.CONFIG.icon_url)[0],
-        "type": "Image",
-        "url": config.CONFIG.icon_url,
-    },
     "publicKey": {
         "id": f"{config.ID}#main-key",
         "owner": config.ID,
@@ -148,6 +143,13 @@ ME = {
     "tag": dedup_tags(_LOCAL_ACTOR_TAGS),
 }
 
+if config.CONFIG.icon_url:
+    ME["icon"] = {
+        "mediaType": mimetypes.guess_type(config.CONFIG.icon_url)[0],
+        "type": "Image",
+        "url": config.CONFIG.icon_url,
+    }
+
 if ALSO_KNOWN_AS:
     ME["alsoKnownAs"] = [ALSO_KNOWN_AS]