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.

docs/reference/glib/gvariant-varargs.xml

@@ -681,6 +681,11 @@ g_variant_unref (value);]]></programlisting></informalexample>
     prefixed with an '<literal>m</literal>', the type of arguments that are collected does not change in any way, but
     <link linkend='NULL--CAPS'><literal>NULL</literal></link> becomes a permissable value, to indicate the Nothing case.
    </para>
+   <para>
+    Note that the "special exception" introduced in the array section for constructing empty arrays is ignored
+    here.  Using a <literal>NULL</literal> pointer with the format string '<literal>mas</literal>' constructs
+    the Nothing value -- not an empty array.
+   </para>
    <para>
     The second way is used with all other format strings.  For
     <link linkend='g-variant-new'><function>g_variant_new()</function></link> an additional