filesystem 2.0 schema check failed: mount_point: expected string, got nothing

[Martin Packman]

Bootstrapping MAAS 2.0 (beta4+bzr4958) on finfolk vmaas fails with the following error:

2016-04-27 17:19:09 ERROR cmd supercommand.go:448 failed to bootstrap model: cannot start bootstrap instance: cannot run instances: cannot run instance: blockdevice 0: partition 0: filesystem 2.0 schema check failed: mount_point: expected string, got nothing

This is apparently because some of the filesystems of an allocated machine are 'null' rather than containing the configuration juju expects:

<http://paste.ubuntu.com/16085749/>

$ maas env20 block-devices read node-ce3fd804-71e4-11e5-80fe-525400c43ce5
[
    {
        ...
        "filesystem": null,
        "used_for": "MBR partitioned with 1 partition",
        ...
    },
    {
        ...
        "filesystem": null,
        "used_for": "Unused",
        ...
    },
    {
        ...
        "filesystem": {
            "uuid": "ac5c61b8-84bb-4b10-b399-466570bae392",
            "mount_point": "/",
            "label": "root",
            "mount_options": null,
            "fstype": "ext4"
        },
        "used_for": "ext4 formatted filesystem mounted at /",
        ...
    }
]