chore: disable null checks until we have time to fix this (#3830)

## About the changes
This action started to fail and despite being safe to ignore it's still
annoying. I'm disabling it for now until I can look into what's going on
with it

Example:
https://github.com/Unleash/unleash/pull/3829#issuecomment-1557372501

.github/workflows/gradual-strict-null-checks.yml

@@ -1,7 +1,7 @@
 name: Lower null checks
 
 on:
-  pull_request:
+  workflow_dispatch:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}