Allow alarm threshold value set to 0

Currently, if we set alarm threshold value to 0, it will return 400
with threshold_rule/threshold is mandatory, which is not suitable.
This patch corrects the check to only fail if mandatory field is
not set or is None.

Change-Id: Id141937337435f41fa7c246d8f0485fdd3bb118b
Closes-Bug: #1313492