Fortran: fix passing of NULL() actual argument to character dummy [PR104819]

Ensure that character length is set and passed by the call to a procedure
when its dummy argument is NULL() with MOLD argument present, or set length
to either 0 or the callee's expected character length.  For assumed-rank
dummies, use the rank of the MOLD argument.  Generate temporaries for
passed arguments when needed.

	PR fortran/104819

gcc/fortran/ChangeLog:

	* trans-expr.cc (conv_null_actual): Helper function to handle
	passing of NULL() to non-optional dummy arguments of non-bind(c)
	procedures.
	(gfc_conv_procedure_call): Use it for character dummies.

gcc/testsuite/ChangeLog:

	* gfortran.dg/null_actual_6.f90: New test.