Sampler's begin for callee's null check before fence (less overhead in corner case)

[vlkale]

This PR is simple fix involving a slight reduction in Kokkos Tools runtime overhead for one of the kokkosp_begin_parallel_xyz callbacks (done correctly for the other two); it does not raise any incorrect behavior in the develop branch version of the sampler. The callee being null is a corner case, and this reduces overhead in that case.

Specifically, in the function kokkosp_begin_parallel_for(), the check of the callee pointer being null is placed after the tool-invoked fence. It should be placed before the tool-invoked fence. If the child callee pointer is null, e.g., the sampler's tool-invoked fence need not be invoked. This would reduce Kokkos Tools runtime overhead.

Note that this is consistent with @(DavidPoliakoff)'s passthrough utility draft PR.