Follow up fixes for the image URL support

app/activitypub.py

@@ -161,6 +161,7 @@ if config.CONFIG.image_url:
         "url": config.CONFIG.image_url,
     }
 
+
 class NotAnObjectError(Exception):
     def __init__(self, url: str, resp: httpx.Response | None = None) -> None:
         message = f"{url} is not an AP activity"

app/config.py

@@ -92,7 +92,7 @@ class Config(pydantic.BaseModel):
     summary: str
     https: bool
     icon_url: str
-    image_url: str
+    image_url: str | None = None
     secret: str
     debug: bool = False
     trusted_hosts: list[str] = ["127.0.0.1"]