backends: Work around a Vala const-array null-termination bug

Bug #682698 means that Vala does not null-terminate const arrays, yet it
expects them to be null terminated when returning them to g_object_get().
This results in g_object_get() walking off into arbitrary memory, and Bad
Things happen shortly afterwards.

Work around this (because it doesn’t look like the Vala bug is going to be
fixed any time soon) by manually null-terminating all const arrays (typically
linkable_properties and writeable_properties lists in backends) and modifying
code which iterates over them to ignore null entries.

https://bugzilla.gnome.org/show_bug.cgi?id=682698