[PRD-2288] Fix null value in 'mac' or 'id'

Check 'mac' and 'id' fields of agent request data for emptiness.
Do not overwrite 'id' for node.
Do not allow 'mac' == None
Tests added.

Change-Id: I8d31d7669eaf78e8c40022b21ef2387723f66d45
Merge "[PRD-2288] Fix null value in 'mac' or 'id'"