bug: ResetTargetsDelegations changes delegations.roles to null value: deserialization error

[asraa]

Currently, if you use repo.ResetTargetsDelegations, the targets delegation looks like:

    "delegations": {
      "keys": {},
      "roles": null
    }

The null in the roles field, while valid JSON, is not handled properly by other TUF clients. For example, python-tuf will through a deserialization error:

Traceback (most recent call last):
  File "/home/asraa/git/python-tuf/tuf/api/serialization/json.py", line 37, in deserialize
    metadata_obj = Metadata.from_dict(json_dict)
  File "/home/asraa/git/python-tuf/tuf/api/metadata.py", line 198, in from_dict
    signed=cast(T, inner_cls.from_dict(metadata.pop("signed"))),
  File "/home/asraa/git/python-tuf/tuf/api/metadata.py", line 1960, in from_dict
    delegations = Delegations.from_dict(delegations_dict)
  File "/home/asraa/git/python-tuf/tuf/api/metadata.py", line 1695, in from_dict
    return cls(keys_res, roles_res, succinct_roles_info, delegations_dict)
  File "/home/asraa/git/python-tuf/tuf/api/metadata.py", line 1630, in __init__
    raise ValueError("One of roles and succinct_roles must be set")
ValueError: One of roles and succinct_roles must be set

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/asraa/git/python-tuf/client-test.py", line 27, in <module>
    updater.refresh()
  File "/home/asraa/git/python-tuf/tuf/ngclient/updater.py", line 129, in refresh
    self._load_targets(Targets.type, Root.type)
  File "/home/asraa/git/python-tuf/tuf/ngclient/updater.py", line 395, in _load_targets
    delegated_targets = self._trusted_set.update_delegated_targets(
  File "/home/asraa/git/python-tuf/tuf/ngclient/_internal/trusted_metadata_set.py", line 417, in update_delegated_targets
    new_delegate = Metadata[Targets].from_bytes(data)
  File "/home/asraa/git/python-tuf/tuf/api/metadata.py", line 264, in from_bytes
    return deserializer.deserialize(data)
  File "/home/asraa/git/python-tuf/tuf/api/serialization/json.py", line 40, in deserialize
    raise DeserializationError("Failed to deserialize JSON") from e
tuf.api.serialization.DeserializationError: Failed to deserialize JSON

Let's fix this to use an empty array instead? Or simply remove the entire delegations. This fixes the problem:

diff --git a/repo.go b/repo.go
index cce0020..8b38d50 100644
--- a/repo.go
+++ b/repo.go
@@ -698,8 +698,7 @@ func (r *Repo) ResetTargetsDelegationsWithExpires(delegator string, expires time
                return fmt.Errorf("error getting delegator (%q) metadata: %w", delegator, err)
        }
 
-       t.Delegations = &data.Delegations{}
-       t.Delegations.Keys = make(map[string]*data.PublicKey)
+       t.Delegations = nil
 
        t.Expires = expires.Round(time.Second)

with thedelegations JSON object removed.

cc @ethan-lowman-dd @joshuagl

Detected in sigstore/root-signing#410