make CTFE Bson.empty{Object,Array} work with old D
make CTFE Bson.empty{Object,Array} work with old D

This copies some logic from the opAssign methods, but given how useful
init objects for CTFE may be, this is a good trade-off

gedaiu/vibe.d,etcimon/vibe.d,etcimon/vibe.d,Geod24/vibe.d,chalucha/vibe.d,rejectedsoftware/vibe.d