Bugfix: return nearest non-null point on interaction when spanGaps=true (#11986)

* First step in fixing the bug of spanGaps null point interaction

* Complete bugfix of spanGaps null point interaction

* Add two tests in core.interaction.tests for the bugfix change

* Remove odd line break

* Use isNullOrUndef helper for point value checks

* Add 10 more test cases for nearest interaction when spanGaps=true