Fix misleading boolean 'null' interval tests

[pepijnve]

Which issue does this PR close?

• None, simply corrects two unit tests

Rationale for this change

The test_and_null_boolean_intervals and test_or_null_boolean_intervals are a bit misleading. They try to create a 'null' interval using Interval::try_new(ScalarValue::Boolean(None), ScalarValue::Boolean(None)). The implementation of try_new normalises this to Interval::UNCERTAIN, which is not the same as 'null'.

What changes are included in this PR?

Adds tests demonstrating:

• Interval::try_new(ScalarValue::Boolean(None), ScalarValue::Boolean(None)) == Interval::UNCERTAIN
• Interval::UNCERTAIN contains ScalarValue::Boolean(Some(true)) and ScalarValue::Boolean(Some(false))
• Interval::UNCERTAIN does not contain ScalarValue::Boolean(None) or ScalarValue::Null

Renames test_and_null_boolean_intervals and test_and_null_boolean_intervals

Are these changes tested?

Test only changes

Are there any user-facing changes?

No

[alamb]

Thank you @pepijnve