IS_NULL should be allowed against nested message fields

[MMcM]

Currently validation and evaluation fail, but this operation seems well-defined.

I do not believe that there is ever an index that can be used for this operation.

If the message has a repeated field, IS_NULL is still disallowed without fan-out, since that seems less well-defined. Also, as noted recently, an index on that repeated field would not put any entry into the index if the message were null or the repeated field were empty.