meta: Warn if a meta implementation is registered without init function

meta: Warn if a meta implementation is registered without init function

This previously caused uninitialized memory unless something else was
initializing all the fields explicitly to something.

To be on the safe side, we also allocate metas without init function to all
zeroes now as it was relatively common.

https://bugzilla.gnome.org/show_bug.cgi?id=764902