Check snapshot value for None

For consistency with the rest if the checks.

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

tuf/ngclient/_internal/metadata_bundle.py

@@ -359,7 +359,7 @@ def update_snapshot(self, data: bytes):  # pylint: disable=too-many-branches
 
         # If an existing trusted snapshot is updated,
         # check for a rollback attack
-        if self.snapshot:
+        if self.snapshot is not None:
             for filename, fileinfo in self.snapshot.signed.meta.items():
                 new_fileinfo = new_snapshot.signed.meta.get(filename)