Fix gitaly cluster test

What does this MR do?

Fixes an undefined method '[]=' error by adding an empty hash to the array of node data if it doesn't include all 3 nodes.

See #295192 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content