[2.0.2 - OSS] UI Alerts : 0 threshold is not set on edit or clone => can't save it without resetting the 0 value

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Create an alert for disk space usage with a OK threshold being between 0 and 80%
2. Edit or clone the alert
3. The 0 value is no longer present - so you can't save the alert till you set the 0 again


__Expected behavior:__

0 should remain saved and set instead of value being empty.

__Actual behavior:__

Field is empty and I had to open browser console to see that a value was missing and that 0 vanished.

It was reported in #17916 and #17729 but it seems not fully fixed yet.

__Environment info:__

* InfluxDB version: 2.0.2 OSS

Reproduce the issue by creating a disk-space alert with an OK threshold of 0, then edit or clone it and inspect the threshold field. The fix is complete when 0 remains populated and the alert can be saved without re-entering it.

Written by the indexing model from the issue text.