[mlir][GPU] treat the absence of workgroup attributes correctly

The helper function in GPUFuncOp incorrectly assumed the workgroup
attribution attribute is always present. Instead, treat its absence as
if its value was zero, i.e., no workgroup attributions are specified.

Closes #58045.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D134865