fix jwilkes/purr-data#18

allow this to work:
gui_start_vmess("foo", "");
gui_start_array();
gui_s("hello");
gui_end_array();
gui_end_vmess();

In other words, a gui message with a single array argument. There should rarely be a need for this (if at all), but it should still send a well-formed message to the GUI (which it wasn't before this bug fix)