refactor(runloop) do not select Services/Routes with nulls
refactor(runloop) do not select Services/Routes with nulls

When reading from the proxy, we do not wish to retrieve unset columns as
`ngx.null` for backwards-compatibility reasons. That is now the default
for all plugins since 735314cf (parent commit), for plugins, and it is
now for the runloop as well.

Context: retrieving unset columns as `nil` instead of `ngx.null` allows
for backwards-compatibility with existing plugins. An example of this is
authentication plugins retrieving the authenticated Consumer, and
injecting its fields as headers into the upstream request (e.g.
`X-Consumer-Username`). When such fields are unset, the following
headers were sent upstream: `X-Consumer-Username: userdata: NULL`
instead of no header at all (or worse, producing HTTP 500 errors).

Mashape/kong,Kong/kong,Kong/kong,Kong/kong