handleMappingRule from DatasetMongoRepository should throw proper error on conformance receiving null

[Zejnilovic]

Describe the bug

When handleMappingRule from DatasetMongoRepository method receives null in dataset.conformance it throws a NullPointer in users face. This can happen when a user sends a null conformance rule value to update of the dataset, although this is a valid value since you don't have to have anything in conformance.

To Reproduce

Steps to reproduce the behavior OR commands run:

1. Try updating dataset through rest and leave out conformance or set it to null
2. Throws 500

Expected behaviour

Throw propper error

Even better. Accept null as an empty array

[Zejnilovic]

@lokm01, @yruslan & @GeorgiChochov -> What do you think? Accept null/undefined or enforce the empty array and give it a proper exception?

EDIT: Adding Georgi since he is back from his sick leave