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.

scripts/config_wizard.py

@@ -75,9 +75,10 @@ def main() -> None:
         proto = "http"
 
     print("Note that you can put your icon/avatar in the static/ directory")
-    dat["icon_url"] = prompt(
+    if icon_url := prompt(
         "icon URL: ", default=f'{proto}://{dat["domain"]}/static/nopic.png'
-    )
+    ):
+        dat["icon_url"] = icon_url
     dat["secret"] = os.urandom(16).hex()
 
     with config_file.open("w") as f: