Panic when listing notifications if project doesn't exist

[shreddedbacon]

When attempting to list notifications for a project, if the project doesn't exist (eg, accidental typo of a project name), the returned data does not contain an error, just a null payload.

{"data":{"projectByName":null}}

When this is called https://github.com/uselagoon/lagoon-cli/blob/main/cmd/notificationsslack.go#L175, the data for result is empty, and result.Notifications is nil which when it tries to iterate over, results in a panic.

Unfortunately the projectByName resolver doesn't seem to return an error if a project doesn't exist :hide-the-pain:
A quick one would be to check that results.Notifications is not nil, and potentially update the query in machinery to also request the name and id fields to check if a project is returned.

This occurs on all list project notifications, not just slack.