evaluate testability for "skip nulls" on ToStringFormatTest

**context**
IDEs offer the feature to skip null values when generating the toString method.

currently the ToStringNullSafetyTest already verifies that the method can handle null values, but we can not assert if the null values will be contained in the result.

**questions**
- does it make sense to test that?
- is that even possible?
- is it convenient?
