bug: fix notifications when not using thresholds

[svenvg93]

📃 Description

Fix notifications not being sent when thresholds are not used.
Please double check the logic :)

The original code used ! $result->healthy which means:

When healthy is null, ! null evaluates to true, so it would skip sending notifications ❌
When healthy is false, ! false evaluates to true, so it would skip ✅
When healthy is true, ! true evaluates to false, so it would send ✅

The fix uses $result->healthy === false which means:
When healthy is null, it will send notifications ✅
When healthy is false, it will skip ✅
When healthy is true, it will send ✅

closes #2456