fortran: [PR121472] Fix ICE with constructor for finalized zero-size type.

fortran: [PR121472] Fix ICE with constructor for finalized zero-size type.

When a derived type has a final subroutine and a constructor interface,
but is effectively zero-sized, the gimplifier fails on the finalization
code.  The existing check for empty types (!derived->components) only
catches completely empty types, not types with empty components.
Replace with a tree-level TYPE_SIZE_UNIT check that catches all
zero-size cases.

	PR fortran/121472

gcc/fortran/ChangeLog:

	* trans.cc (gfc_finalize_tree_expr): Replace !derived->components
	check with TYPE_SIZE_UNIT check for zero-size types.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr121472.f90: New test.