Nullability of texSubImage entrypoints vs compressedTex[Sub]Image

We tried making texSubImage entrypoints non-nullable, instead of generating INVALID_VALUE, but reverted back to using INVALID_VALUE because of the implicit webidl coersion of `null` to `0`, which selects the PBO offset upload entrypoint.

However, compressedTex[Sub]Image has this same problem, since it was non-nullable already in WebGL 1.

I would rather change texSubImage back to non-nullable to have the webidl match intent, rather than add nullable entrypoints to compressedTex[Sub]Image to match behavior.

I propose that we detect when this null-coersion is likely happening, and when we generate INVALID_OPERATION for using the PBO offset entrypoint without a PBO bound, we suggest that perhaps their ArrayBufferView is null.

Start by tracing the texSubImage and compressedTex[Sub]Image entrypoints, then find the INVALID_OPERATION path for PBO offset uploads. Done means detecting likely null coercion and suggesting that the caller's ArrayBufferView may be null, while preserving the intended entrypoint behavior.

Written by the indexing model from the issue text.