Fortran: no size check passing NULL() without MOLD argument [PR55978]

gcc/fortran/ChangeLog:

	PR fortran/55978
	* interface.cc (gfc_compare_actual_formal): Skip size check for
	NULL() actual without MOLD argument.

gcc/testsuite/ChangeLog:

	PR fortran/55978
	* gfortran.dg/null_actual_5.f90: New test.