Use TrustedMetadataSet.root property consistently

In order to consistently use the 'root' property internally,
just try/except the KeyError during the loading of initial
trusted root when 'root' key does not exist yet.

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

tuf/ngclient/_internal/trusted_metadata_set.py

@@ -169,14 +169,16 @@ def update_root(self, data: bytes) -> None:
  f"Expected 'root', got '{new_root.signed.type}'"
  )
 
- if self._trusted_set.get("root") is not None:
+ try:
  # We are not loading initial trusted root: verify the new one
  self.root.verify_delegate("root", new_root)
 
  if new_root.signed.version != self.root.signed.version + 1:
  raise exceptions.ReplayedMetadataError(
  "root", new_root.signed.version, self.root.signed.version
  )
+ except KeyError:
+ logger.debug("Loading initial trusted root")
 
  new_root.verify_delegate("root", new_root)