c++: non-dep structured binding decltype again [PR117107]

The patch for PR92687 handled the usual case of a decomp variable not being
in the table, but missed the case of there being nothing in the table yet.

	PR c++/117107
	PR c++/92687

gcc/cp/ChangeLog:

	* decl.cc (lookup_decomp_type): Handle null table.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/decomp10.C: New test.