iFixit API is returning "null" when listing category

[benoit74]

When listing categories, the API is now returning something like

{
    "Mac": {
        "Mac Hardware": {
            "Apple Time Capsule": {
                "Apple Time Capsule Model A1302": null,
                "AirPort Time Capsule A1470": null,
                "AirPort Time Capsule Model A1409": null,
                "Apple Time Capsule Model A1254": null,
                "Apple Time Capsule A1355 (3.Generation)": null
            }
        }
    }
}

This creates a bug in _process_categories, most probably because the API was previously returning something like this:

{
    "Mac": {
        "Mac Hardware": {
            "Apple Time Capsule": {
                "Apple Time Capsule Model A1302": {},
                "AirPort Time Capsule A1470": {},
                "AirPort Time Capsule Model A1409": {},
                "Apple Time Capsule Model A1254": {},
                "Apple Time Capsule A1355 (3.Generation)": {}
            }
        }
    }
}

[benoit74]

Fixed by https://github.com/openzim/ifixit/commit/a14ce78949d401c74cc7abd85c0c3d1255cb3c38