layout: Never return NULL from pango_layout_get_text()

layouts get initialized with text == NULL as an optimization (avoid a
malloc). But pango_layout_set_text (layout, NULL, 0); will set the text
to "", so it is impossible to set a NULL text.

Fxies crashers in various places that assume NULL return values never
happen.

https://bugzilla.gnome.org/show_bug.cgi?id=707659