fix(auth): guard JWTHandler.is_jwt() against None token

When JWT auth is enabled and a request arrives without an Authorization
header (e.g. health checks, monitoring), api_key is None due to
APIKeyHeader(auto_error=False). The is_jwt() call crashes with
AttributeError: 'NoneType' object has no attribute 'split'.

Return False for None tokens since they are not JWTs.

litellm/proxy/auth/handle_jwt.py

@@ -89,7 +89,9 @@ class JWTHandler:
         self.leeway = leeway
 
     @staticmethod
-    def is_jwt(token: str):
+    def is_jwt(token: Optional[str]):
+        if token is None:
+            return False
         parts = token.split(".")
         return len(parts) == 3