Migrated CIs from split repos should be conditionally required

For example, the CI of [sdk-platform-java](https://github.com/googleapis/google-cloud-java/blob/main/.github/workflows/sdk-platform-java-ci.yaml) is currently not required. This could cause mistakes of merging in changes with failing tests. 

We should make all migrated CIs from split repos conditionally required and always pass if there are no changes to the specific folder.