Pause Behavior: No Condition doesn't work as expected

[zackcl]

Version where bug was found:
6.3.0

Describe the bug
When pause behavior is set to "No Condition" through the UI, the PUT /api/experiments/:id request payload doesn't include the "revertTo": null which results in users still getting conditions when /assign is called.

Expected behavior
The PUT /api/experiments/:id request payload should include "revertTo": null when updating the pause behavior to "No Condition".

[zackcl]

QA: Passed