c++: Fix empty base stores in cxx_eval_store_expression [PR100111]

In r11-6895 handling of empty bases has been fixed such that non-lval
stores of empty classes are not added when the type of *valp doesn't
match the type of the initializer, but as this testcase shows it is
done only when *valp is non-NULL.  If it is NULL, we still shouldn't
add empty class constructors if the type of the constructor elt *valp
points to doesn't match.

2021-04-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/100111
	* constexpr.c (cxx_eval_store_expression): Don't add CONSTRUCTORs
	for empty classes into *valp when types don't match even when *valp
	is NULL.

	* g++.dg/cpp0x/constexpr-100111.C: New test.