GVariant: support NULL for empty arrays in varargs

GVariant: support NULL for empty arrays in varargs

g_variant_new("as", NULL); now gives an empty array of strings, for
example.

This was documented as working already, but was never actually
implemented (due to the fact that it muddies the water when considering
maybe types).  It's being implemented now because its convenience to
programmers exceeds any damage done to the conceptual purity of the API.