[flang] Allow NULL() actual argument for optional dummy procedure

[klausler]

Summary

A disassociated procedure pointer is allowed to be passed as an absent
actual argument that corresponds to an optional dummy procedure,
but not NULL(); accept that case as well.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-call.cpp

               dummyName);
         }
       } else if (IsNullPointer(*expr)) {
-        if (!dummyIsPointer) {
+        if (!dummyIsPointer &&
+            !dummy.attrs.test(
+                characteristics::DummyProcedure::Attr::Optional)) {
           messages.Say(
               "Actual argument associated with procedure %s is a null pointer"_err_en_US,
               dummyName);

flang/test/Semantics/call02.f90

       !ERROR: A dummy procedure may not be ELEMENTAL
       procedure(elem) :: dummy
     end subroutine
+    subroutine optionalsubr(dummy)
+      procedure(sin), optional :: dummy
+    end subroutine
+    subroutine ptrsubr(dummy)
+      procedure(sin), pointer, intent(in) :: dummy
+    end subroutine
   end interface
   intrinsic :: cos
   call subr(cos) ! not an error

   call subr(elem) ! C1533
   !ERROR: Actual argument associated with procedure dummy argument 'dummy=' is a null pointer
   call subr(null())
+  call optionalsubr(null()) ! ok
+  call ptrsubr(null()) ! ok
   !ERROR: Actual argument associated with procedure dummy argument 'dummy=' is typeless
   call subr(B"1010")
 end subroutine