Default fill_min/fill_max to `nil`

fill_min/fill_max in the #vtree{} record are `nil` by default
and will set by the corresponding API (Couchbase or Apache CouchDB).

Change-Id: I11814f6982f5792c3f07f73a4770811e6cc294af
Update for recent Apache CouchDB 1.3.x changes

COUCHDB-1334 [1] was reverted in a8aff9f91790b0dae347b10b7c11b21f34faa0b2 [2],
hence GeoCouch needed to be adapted as well.

[1] https://issues.apache.org/jira/browse/COUCHDB-1334
[2] https://github.com/apache/couchdb/commit/a8aff9f91790b0dae347b10b7c11b21f34faa0b2

Change-Id: I5f31e390accbc27ba9b409b465b8e7d32e26d906