AlterProperty delta fails if altering a property with a Linked Class

[mmacfadden]

Versions

• Convergence Version: 1.0.0-rc.9

Describe the Bug
When using an alter property, the structure is not smart enough to tell if you are unsetting the linked class, or if you are simply choosing not to alter it. This can result in a null pointer exception in OrientDB in the DeltaProcessor.

https://github.com/convergencelabs/convergence-server/blob/66d3f7ec46bfa4cdd1b266802da1e32601a57ea1/src/main/scala/com/convergencelabs/convergence/server/backend/db/schema/DeltaProcessor.scala#L139

At this time we don't have any deltas that expose this bug, or that alter the linked class. We need to change the structure of this delta object tin a backwards compatible way.

[mmacfadden]

Created a fix to work around this OrientDB bug:

orientechnologies/orientdb#9639