Handle edge-cases where an avatar uses an arbitrary model or doesn't have a hip bone.

[Gigabyte5671]

Difficult to say. I did discover that you can currently load any mesh as an avatar even if there is no bone. There might be a time when this is desirable. For example loading in a "speaker" to stream music without having to add bones to this object. This might be a bit of an outlier though. Worth a bit of thought. On the other hand requiring a hip bone may also reduce abuse in the meantime. For example selecting an entire environment and using this as avatar on a whim. On the other hand you may also have an irregular avatar that wouldn't have a "hip" bone.

Originally posted by @Aitolda in https://github.com/vircadia/vircadia-web/pull/130#discussion_r1099031509