Allow start date to be optional

Problem/Motivation

In #2794481: Allow end date to be optional we allow end date to be optional in DateRangeItem. But the start date is required and we should also allow it to be optional because this is a valid use case.

Steps to reproduce

Proposed resolution

Make value(start date) in DateRangeItem optional, add validation constraint via configuration.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet