Drupal\datetime_range\Plugin\Field\FieldType\DateRangeItem::mainPropertyName() should return NULL

Change seems minimal. Searched for mainPropertyName in datetime and datetime_range and didn't find any so think should be good.

This has big repercussions for anybody actually interacting with DateRangeItem fields today though!

As the absolute minimum, this needs a change record.

And more likely: this needs to trigger a deprecation and can only start doing this in the future?

Also, this would be in direct conflict with ✨ Allow end date to be optional Needs review AFAICT? 🤔

Thanks @wim-leers. You are absolutely right. This might break the current behavior of DateRangeItem, in case the "value" is used as main property. I understood the datetime range as a way consistently managing range data so an end date is necessary, if it's not the case then we should just use the datetime field.

And thank you for pointing out the conflict issue #2794481. If that issue is accepted then it further weakens my argument here, we just should not apply this change. But from issue #2794481 I see the change makes the responsibility of datetime and datetime_range even vague, and in my opinion cause more confusion. So I think we should keep this issue open, maybe someone can provide a clearer insight about the design decision.

+1 to keeping this open, and agreed that this field type is rather confusing! 😅