[#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
Update set-permissions in testing docs
Update set-permissions in paster docs
Merge branch '1776-empty-array-json-datastore'
Merge pull request #1845 from morty/set-permissions-doc-update

Set permissions doc update