glib: Make sure that empty `glib::StrV` / `glib::PtrSlice` returns a valid pointer#1734

sdroege merged 2 commits intogtk-rs:maingtk-rs/gtk-rs-core:mainfrom

sdroege:strv-into-raw-emptysdroege/gtk-rs-core:strv-into-raw-emptyCopy head branch name to clipboard