text-art: allow wrapper_widget to have no child

text-art: allow wrapper_widget to have no child

During prototyping a feature I found it useful for
text_art::wrapper_widget to be able to not have a child.

gcc/ChangeLog:
	* text-art/widget.cc (selftest::test_empty_wrapper_widget): New.
	(selftest::text_art_widget_cc_tests): Call it.
	* text-art/widget.h (text_art::wrapper_widget::calc_req_size):
	Gracefully handle m_child being null.
	(text_art::wrapper_widget::update_child_alloc_rects): Likewise.
	(text_art::wrapper_widget::paint_to_canvas): Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>