[#1776] Fix bug when upserting [] on JSON column in the datastore

We were testing that the new value wasn't `None` using `if value`. As `[]` is a
falsy value, we never went through that `if` and ended up trying to run an
invalid SQL query.

This commit fixes that by explictly testing `if value is not None`.

This was first discovered in
http://stackoverflow.com/questions/24207065/inserting-empty-arrays-in-json-type-fields-in-datastore