c++: lambda template in requires [PR105541]

c++: lambda template in requires [PR105541]

Since the patch for PR103408, the template parameters for the lambda in this
test have level 1 instead of 2, and we were treating null template args as 1
level of arguments, so tsubst_template_parms decided it had nothing to do.
Fixed by distinguishing between <> and no args at all, which is what we have
in our "substitution" in a requires-expression.

	PR c++/105541

gcc/cp/ChangeLog:

	* cp-tree.h (TMPL_ARGS_DEPTH): 0 for null args.
	* parser.cc (cp_parser_enclosed_template_argument_list):
	Use 0-length TREE_VEC for <>.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-requires1.C: New test.