Bugfix: null Foreign Keys on /v2/items remain null when depth>0

[calumbell]

Closes #621

This PR addresses a bug on the /v2/items endpoint where the weapon and armor fields where returning inconsistant results when different depth query parameters were passed with API requests. These fields might have returned null at depth=0, but instead returned a large object with mostly empty keys.

Follow this bugfix, null "weapon" and "armor" fields now remain null at all depths.

/v2/items/srd_alchemists-fire-flask/?fields=name,weapon,armor&depth=1

{
    "weapon": null,
    "armor": null,
    "name": "Alchemist's Fire (Flask)"
}

Items with a defined "armor" or "weapon" work as they did before:

/v2/items/srd_longsword/?fields=name,weapon,armor&depth=1

{
    "weapon": {
        "key": "srd_longsword",
        "is_versatile": true,
         ...
    },
    "armor": null,
    "name": "Longsword"
}

/v2/items/srd_chain-mail/?fields=name,weapon,armor&depth=1

{
    "weapon": null,
    "armor": {
        "key": "srd_chain-mail",
        "ac_display": "16",
        "category": "heavy",
        ...
    },
    "name": "Chain mail"
}