Merge undefineds into undefined rather than []

ve.dm.MetaLinearData.static.merge( [ undefined, undefined, ... ] )
returned [ [] ], make it return [ undefined ] instead. This allows
us to lazily use the merge function to create metadata without having
to worry about denormalizing undefined to [] all over the place.

Change-Id: I41fe6472c2185a6315119a546a73765966d013d3
Merge "Merge undefineds into undefined rather than []"