[FIX] Implemented logic to handle cases where no diagnosis/assessment exist in the graph

[rmanaem]

• Closes Handle case where no diagnosis/assessment exist in the graph #328

Checklist

• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see https://neurobagel.org/contributing/pull_requests for more info)
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

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[surchs]

Very nice @rmanaem, tests look good, other stuff makes sense. I left some 🍒 comments for you to look at, but good to go 🧑‍🍳

One thing to check before merging: in the staging build I cannot see any toast appear, even though the assessment and diagnosis are both empty. We should make sure this is expected - and then fix this.