typing: Apply trivial none-checks with assertions as necessary.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

zerver/lib/camo.py

@@ -6,6 +6,7 @@ from django.conf import settings
 
 def generate_camo_url(url: str) -> str:
     encoded_url = url.encode()
+    assert settings.CAMO_KEY is not None
     encoded_camo_key = settings.CAMO_KEY.encode()
     digest = hmac.new(encoded_camo_key, encoded_url, hashlib.sha1).hexdigest()
     return f"{digest}/{encoded_url.hex()}"