still set originalStart when start is absent
still set originalStart when start is absent

fixes CNVS-25122

set it to null, instead of skipping the assignment

test-plan:
 - create an dated event (start is present)
 - edit the event to make it undated (start becomes absent)
 - after event is saved to server, it should properly move to the
   undated section of the page

Change-Id: I6a7dc6d60d6ef072b6785f373feb13bb01cebcfa
Reviewed-on: https://gerrit.instructure.com/67548
Reviewed-by: Matthew Wheeler <ce71fbafda91d6c8f12582e1837687d5fe592dd9@instructure.com>
Tested-by: Jenkins
QA-Review: Derek Hansen <cb1116771307a93b98b6babf698410ed3846296d@instructure.com>
Product-Review: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>

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