[mlir][Vector] Support 0-D vectors in `CreateMaskOp`

[mlir][Vector] Support 0-D vectors in `CreateMaskOp`

The 0-D case gets lowered in almost the same way that the 1-D case does
in VectorCreateMaskOpConversion. I also had to slightly update the
verifier for the op to always require exactly 1 operand in the 0-D case.

Depends On D115220

Reviewed by: ftynse

Differential revision: https://reviews.llvm.org/D115221