[App] Fix bug when preventing cross launch of apps

[luca3rd]

What does this PR do?

Fixes bug introduced in #15226.

If you launch an app with lightning run app app.py --cloud, --cluster-id was not used so cluster_id is None. If the app has never ran before, and there are no BYOC clusters in the default project, the app will run on the global lightning cluster.

The bug occurs when launching the same app a second time using the same command. We checked to see if the cluster_id matches the cluster of the previous app instance (which was set by the platform to the global cluster). Essentially it was checking if None == 'lightning-cluster'.

This PR fixes the bug by setting cluster_id to the cluster of the previous instance, if it was not set by the user.

To run through some examples and desired behaviour:

Scenario 1

1. lightning run app app.py --cloud -> creates the app on the global lightning cluster
2. lightning run app app.py --cloud -> updates the app instance on the global cluster

Scenario 2

1. lightning run app app.py --cloud --cluster-id my-cluster -> creates the app on 'my-cluster'
2. lightning run app app.py --cloud -> updates the app instance on 'my-cluster'

Scenario 3

1. lightning run app app.py --cloud --cluster-id my-cluster -> creates the app on 'my-cluster'
2. lightning run app app.py --cloud --cluster-id some-other-cluster -> Error: we don't support 1 app on multiple clusters (yet). See PR description in Prevent bug when launching apps on multiple clusters #15226.

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