[518983] CompareInputAdapter should cache ancestor, left, and right

Avoid recomputing the ancestor, left, and right side of a compare input
adapter:
- Cache value once it is retrieved
- Use special null element to preserve existing null behavior

Bug: 518983
Change-Id: Ib6fafdf0a190b3e1e72060bb47dd1e959a55858e
Signed-off-by: Martin Fleck <mfleck@eclipsesource.com>
1 file changed