Tolerate null on Json db load

A icehouse->juno migration issue has surfaced where
resource.properties_data is null for existing resource records.

This change fixes the Json db type to treat None the same as
the serialized JSON 'null'.

Change-Id: Ia0b9a23a7798c95fbbcba439ffb8203b50367e7c
Closes-Bug: #1383505