[mlir][Linalg] Handle null affine map returns from inversePermutation.

The inversePermutation method returns a null map on failure. Update
uses of this method within Linalg to handle this. In LinalgToLoops the
null return value was used to emit scalar code. Modify that to return
failure, and emit scalar implementation when affine map is "empty",
i.e. 1 dims, 0 symbols and no result exprs.

Differential Revision: https://reviews.llvm.org/D77964