MDM throws nonsensical error when updating resources after $mdm-clear

[jdar8]

Describe the bug
This issue requests better error handling for an invalid condition: an $mdm-clear, no $mdm-submit, and updating existing resources.
The $mdm-clear operation "is intended to be used during the rules-tuning phase of the MDM implementation" which typically involves performing a $mdm-clear, tuning rules, then a $mdm-submit. If an update to an existing resource is performed after the $mdm-clear, but before the $mdm-submit, there will be a NullPointerException.

To Reproduce
Steps to reproduce the behavior:

1. Post mdm bundle
2. Perform $mdm-clear
3. Post the bundle again (with some edits to make it slightly different)

Expected behavior
Better error handling in this edge case scenario where a resource is updated between a $mdm-clear and $mdm-submit