Gracefully handle items with no texture (default replacement item texture?)

[deathcap]

getItemTexture() will first try to use the itemTexture property, then fallback to the block texture if there isn't one, but if neither are specified, _getTextureURLs() will will be called with textureName = undefined:

causing a null dereference on names.top in _getTextureURLs(). This case should be better handled, perhaps by returning a default item texture (similar to the default "missing block" texture), instead of crashing.

(above screenshot occurred with voxel-clientmc ef8aa8f, logging into a server with a player holding wheat seeds)