handleMappingRule from DatasetMongoRepository should throw proper error on conformance receiving null

## 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