[MLIR] Handle corner case in MaterializeVectors

This corner was found when stress testing with a functional end-to-end CPU
path. In the case where the hardware vector size is 1x...x1 the `keep` vector
is empty and would result a crash.

While there is no reason to expect a 1x...x1 HW vector in practice, this case
can just gracefully degrade to scalar, which is what this CL allows.

PiperOrigin-RevId: 227761097