Add tests for required fields

[vkrasnovyd]

• Create base test case that verifies that it's impossible to set a required field to None. Use handle_separately for better performance.
• Add test with the new test case for each model that has requied fields.

[vkrasnovyd]

Separate check for each collection is no longer needed with rel-db.
However, test for handling empty required fields by database writer covers only the create operations.
Tests similar to test_create_error_own_field_not_null should be created also for update operation.