[mlir][sparse] preserve zero-initialization for materializing buffers

This revision makes sure that when the output buffer materializes locally
(in contrast with the passing in of output tensors either in-place or not
in-place), the zero initialization assumption is preserved. This also adds
a bit more documentation on our sparse kernel assumption (viz. TACO
assumptions).

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D110442