Support nullptr value of argument ptr_C for xe_array_epilogue

[sanchitintel]

Problem

In a real-use case of GroupGEMM, we may not have any C matrices. When beta is 0, passing a bogus ptr_C argument for xe_array_epilogue, such as ptr_D, if they are the same dtype, still results in some wasteful compute, so for the default Group GEMM example (examples/04_bmg_grouped_gemm.cpp), there's a slowdown of ~0.1 TFLOPs.

Solution

Preferable solution is to use beta=0 & a non-null value for ptr_C, such as ptr_D, e.g. static_cast<const ElementC**>((void*)ptr_D.get()) even when C & D are not the same dtype because C tiles aren't actually used when beta is 0.

However, in this implementation in this PR, if a user still wants to pass nullptr argument for ptr_C, then C should not be used irrespective of whatever the beta value is.

Given that the aforementioned workaround doesn't result in a discernible performance penalty, I'm not sure if this PR makes sense.