Fix ci_url might be None
# What does this PR do?

In `notification_service.py`, we have

```
    ci_url = os.environ.get("CI_COMMIT_URL")

    commit_number = ci_url.split("/")[-1]
```
but `ci_url` might be `None`, for example, for scheduled CI.

This PR moves the involved block inside 

```
    if ci_title is not None:
        assert ci_url is not None
```
(i.e. only for push CI)
_The documentation is not available anymore as the PR was closed or merged._