Add nullable annotation to combo_box_text_get_active_text

This missing nullable annotation causes haskell-gi to generate a binding that doesn't wrap the return value in a Maybe monad, meaning Haskell code using it crashes with a null pointer exception when you try to read a ComboBoxText before the user selects an option. I at first thought this was a bug in haskell-gi and discussed the issue over there at issue #174, and was led to the belief that Haskell-gi's bindings only have that problem because of this missing annotation, and while it would be easy to special-case their package to override this, that would be a bandaid fix and would not propagate the benefit to any other packages relying on GObject Introspection to generate bindings to Gtk. Given that the situation there isn't urgent, we decided it would be best to submit this here.