Fix annotation breakage

Inez reported that unitalicizing from the toolbar was broken, because
the toolbar was somehow generating annotations that had .attributes={}
as opposed to .attributes=undefined. Turned out the cause was in the
default value for element in the ve.dm.Model constructor.

Change-Id: I64ea9ef56cd15d1131c1aa23484d7420c95a8225
Merge "Fix annotation breakage"