The gtk_builtin_icon_get_default_size_property returns a const char *,
in a way such that some compilers insist that something that is of a
pointer value be returned, so fix that by replacing 0 with NULL.
Make min-width/height have preference over the set default size. This
allows shrinking the widget. The default size is only used if min-width
is not set (or explicitly set to 0.