When querying on distance to reference coordinates, and when some document misses a value for the location field:

• the Lucene backend computes the distance between the reference coordinates and (0,0)
• the ElasticSearch backend simply returns a distance of Double.MAX_VALUE (~1*10^308)

This is illustrated by this commit, where I had to adapt a test in order to take into account the inconsistency between the two backends.

I think we should make the behavior consistent.
IMO, the ElasticSearch backend's behavior is more sensible. Changing the Lucene backend's behavior could be considered as a breaking change. On the other hand, changing the behavior of ES might prove difficult.