Check relations: `null` not found in resource

[mike-podolskiy90]

When tried to do checkRelations got this error message:

Foreign key validation failed: [parentEventID] -> [eventID]: 'null' not found in resource 'event'.

It looks like it's trying to find nulls ​​in the referenced resource. I guess we should just skip nulls. I'll submit a PR.

Related #4

[iSnow]

Thanks for the issue and PR. I've merged it and extended the test data to cover this.

Take care, I removed the PrimaryKeysTest.java and recreated it under fk/