Make TrustedMetadataSet.root non-optional

The 'root' property is guaranteed to be set after init.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Author: sechkova

tuf/ngclient/_internal/trusted_metadata_set.py

@@ -156,9 +156,9 @@ def __iter__(self) -> Iterator[Metadata]:
 
     # Helper properties for top level metadata
     @property
-    def root(self) -> Optional[Metadata]:
-        """Current root Metadata or None"""
-        return self._trusted_set.get("root")
+    def root(self) -> Metadata:
+        """Current root Metadata"""
+        return self._trusted_set["root"]
 
     @property
     def timestamp(self) -> Optional[Metadata]:
@@ -205,7 +205,7 @@ def update_root(self, data: bytes) -> None:
                 f"Expected 'root', got '{new_root.signed.type}'"
             )
 
-        if self.root is not None:
+        if self._trusted_set.get("root") is not None:
             # We are not loading initial trusted root: verify the new one
             if not verify_with_threshold(self.root, "root", new_root):
                 raise exceptions.UnsignedMetadataError(