Return empty collections on error from repeatable ChangeFields

If the change is no longer in the changes table or the commit of
the current patch set is unavailable to be parsed FieldDefs were
returning a null collection to the index implementation. This
violates the FieldDef.Repeatable contract. A field must return a
non-null collection for iteration by the index's document builder.

Return empty collections from these two cases matching behavior
with all other repeatable fields.

Change-Id: I42217970312675b9c35350ba2eb281f5070b7a82
1 file changed