InlineDataFlowController credentials checking and error handling

[ndr-brt]

Currently, on main branch, the sample 04 is not working because this error:
Did not find credentials for data destination.

This is thrown when the data destination key name is null.
At this point I have some questions:

• should we check also the secret value?
• are we supposed to support no-credentials transfers? because at the moment, on sample-04 leans on the NullVault that gives a null credential and the transfer process is working. if this is ok I think that null-check on the keyName should not fail the transfer.
• not directly related: is correct that on that kind of error the ERROR_RETRY is returned? I think that is a thing that should be fail fatally and fixed with a brand new transfer