Check the format of keyval in Root

Signed-off-by: Martin Vrachev <[email protected]>

Author: MVrachev

tuf/api/metadata.py

@@ -425,6 +425,8 @@ def __init__(
         keyval: Mapping[str, str],
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
+        if not keyval.get("public"):
+            raise ValueError(f"keyval doesn't follow the specification format!")
         self.keytype = keytype
         self.scheme = scheme
         self.keyval = keyval