Adapt null resource to be treated as reference instead of empty string.

[iBotPeaches]

This now decodes into:

    <color name="green_light_gradient">#ff52cb8c</color>
    <color name="green_oc831f">@null</color>
    <color name="grey">#ffd0cecd</color>

Instead of either crashing or becoming an <item>

Fixes: #3416