Prevent unnecessary displayName updates in data-sink-worker

[garrrikkotua]

Changes proposed ✍️

What

🤖 Generated by Copilot at 3534e0c

Improved logic for setting member display name in member.service.ts. Prevented overwriting existing display name when processing message queue data.
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🤖 Generated by Copilot at 3534e0c

displayName set
No need to update again
Save user's winter

Why

How

🤖 Generated by Copilot at 3534e0c

• Prevent overwriting display name in database by checking if it is already set (link)

Checklist ✅

• Label appropriately with Feature, Improvement, or Bug.
• Add screehshots to the PR description for relevant FE changes
• New backend functionality has been unit-tested.
• API documentation has been updated (if necessary) (see docs on API documentation).
• Quality standards are met.