GtkListBox: Allow a NULL callback in bind_model
GtkListBox: Allow a NULL callback in bind_model

When unsetting the bound model, one can pass NULL, but in that case the
create_func parameter should also be NULL.

jigpu/gtk,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,chergert/gtk,jigpu/gtk,jadahl/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,chergert/gtk,davidgumberg/gtk,davidgumberg/gtk,grubersjoe/adwaita,chergert/gtk,chergert/gtk,jigpu/gtk,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,alexlarsson/gtk,jadahl/gtk,jadahl/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk