made cxx_builtin_include_directories also support an empty key

as title, hope most attr can support empty key like toolchain_roots attr

Start by locating the definitions and handling for cxx_builtin_include_directories and toolchain_roots. Compare how toolchain_roots accepts an empty key, then determine the expected equivalent behavior for cxx_builtin_include_directories. Done means the empty-key case is supported consistently and covered by the relevant tests, though no test file is named in the issue.

Written by the indexing model from the issue text.