Make TrustedMetadataSet.root non-optional

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

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

tuf/ngclient/_internal/trusted_metadata_set.py

@@ -120,9 +120,9 @@ 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]:
@@ -169,7 +169,7 @@ 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
  self.root.verify_delegate("root", new_root)