[OpenMP] libomp: add check of task function pointer for NULL.

This patch allows to simplify compiler implementation on "taskwait nowait"
construct. The "taskwait nowait" is semantically equivalent to the empty task.
Instead of creating an empty routine as a task entry, compiler can just send
NULL pointer to the runtime. Then the runtime will make all the work with
dependences and return because of the absent task routine.

Differential Revision: https://reviews.llvm.org/D112015