cssshadowsvalue: handle gtk_css_value_transition returning NULL

cssshadowsvalue: handle gtk_css_value_transition returning NULL

The implementation of transition for GtkCssShadowValue can return NULL
at least when the two values have a different inset; all other parts of
the GTK/CSS machinery (e.g. GtkCssArrayValue) handle this by returning
NULL too. Instead, GtkCssShadowsValue was returning an invalid value,
where "len" was set, but some values in the array were NULL, which would
lead to a segfault when this value is later evaluated by the compute
function.

Fix this by making GtkCssShadowsValue return NULL if a shadow transition
fails, like GtkCssArrayValue does.

https://bugzilla.gnome.org/show_bug.cgi?id=686013