fix(API): client accepts empty signatures

ape_safe/client/__init__.py

@@ -123,7 +123,7 @@ def post_transaction(
                 b"",
             )
         )
-        post_dict: Dict = {"signature": signature.hex()}
+        post_dict: Dict = {"signature": signature.hex() if signature else None}
 
         for key, value in tx_data.model_dump(by_alias=True, mode="json").items():
             if isinstance(value, HexBytes):