util: don't return a placeholder netdef in the iterator

[daniloegea]

Description

Tripped on this while working on netplan diff. We probably want to patch the netplan package as well.

Traceback (most recent call last):
  File "/root/netplan/tools/diff.py", line 117, in <module>
    diff = diff_state.get_diff()
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/share/netplan/netplan_cli/cli/state_diff.py", line 149, in get_diff
    full_state = self.get_full_state()
                 ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/share/netplan/netplan_cli/cli/state_diff.py", line 103, in get_full_state
    device_type = DEVICE_TYPES.get(config.type, 'unknown')
                                   ^^^^^^^^^^^
  File "/usr/local/lib/python3.11/dist-packages/netplan/netdef.py", line 118, in type
    return ffi.string(lib.netplan_def_type_name(lib.netplan_netdef_get_type(self._ptr))).decode('utf-8')
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: cannot use string() on <cdata 'char *' NULL>

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

Thanks, lgtm!

Ubuntu is in feature freeze already, so we'd need a bug report on Launchpad, which we can reference/close, when uploading this as a distro patch to the packaging. Could you create one?

[daniloegea]

LP bug created https://bugs.launchpad.net/ubuntu/+source/netplan.io/+bug/2034024