Allow MPI_PROC_NULL for RMA tests

The first commit removes test cases where MPI_PROC_NULL is considered an erroneous value for RMA communication. As of MPI Standard 4.1, p. 567:

MPI_PROC_NULL is a valid target rank in all MPI RMA communication calls. The effect
is the same as for MPI_PROC_NULL in MPI point-to-point communication.

The second commit adds correct test cases using that value; if requested I can drop it.