Failure to process commissioning results with memory in NUMA node with no CPU

Whilst processing commissioning results gathered from a PPC64 machine with NUMA nodes for memory that don't have corresponding CPU cores, MAAS hits a traceback.

MAAS 2.8.0

        Traceback (most recent call last):
          File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 800, in signal
            target_status = process(node, request, status)
          File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 622, in _process_commissioning
            node, node.current_commissioning_script_set, request, status
          File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 515, in _store_results
            **args, timedout=(status == SIGNAL_STATUS.TIMEDOUT)
          File "/usr/lib/python3/dist-packages/metadataserver/models/scriptresult.py", line 391, in store_result
            exit_status=self.exit_status,
        --- <exception caught here> ---
          File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 441, in try_or_log_event
            func(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/hooks.py", line 721, in process_lxd_results
            _process_lxd_resources(node, data["resources"])
          File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/hooks.py", line 452, in _process_lxd_resources
            node.memory, numa_nodes = _parse_memory(data.get("memory", {}), numa_nodes)
          File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/hooks.py", line 490, in _parse_memory
            memory_node.get("total", 0) / 1024 ** 2
        builtins.KeyError: 252

Snippet for context
    "environment": {
        "kernel": "Linux",
        "kernel_architecture": "ppc64le",
        "kernel_version": "4.15.0-106-generic",
        "os_name": "ubuntu",
        "os_version": "18.04",
        "server": "maas-machine-resources",
        "server_name": "bobone",
        "server_version": "4.1"
    },