[FIX] Handle empty local nodes file

[alyssadai]

• Closes Add test case for empty local_nb_nodes.json file #66

Changes proposed in this pull request:

• Empty local_nb_nodes.json file now handled gracefully without erroring
• Added new test case for above
• Moved test case for local nodes file with empty dict {} to more appropriate test (as it currently raises a warning)

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 has a label for the release changelog or skip-release (to be applied by maintainers only)
• 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.

[coveralls]

Pull Request Test Coverage Report for Build 7729709162

• 0 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 99.322%

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Totals
Change from base Build 7722223666:	0.005%
Covered Lines:	586
Relevant Lines:	590

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💛 - Coveralls

[surchs]

looks very nice! 🧑‍🍳

[surchs]

🚀 PR was released in v0.1.0 🚀