Add type check

mypy warns about this but we know that encode_canonical() cannot return
None if we don't set output_function argument.

ruff does not like assert so I added a "noqa" and a comment

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

tuf/api/serialization/json.py

@@ -98,7 +98,10 @@ def serialize(self, signed_obj: Signed) -> bytes:
         """
         try:
             signed_dict = signed_obj.to_dict()
-            canonical_bytes = encode_canonical(signed_dict).encode("utf-8")
+            canon_str = encode_canonical(signed_dict)
+            # encode_canonical cannot return None if output_function is not set
+            assert canon_str is not None  # noqa: S101
+            canonical_bytes = canon_str.encode("utf-8")
 
         except Exception as e:
             raise SerializationError from e