[MM-11974] Change default return string for channel displayname

[sudheerDev]

Ticket Link

MM-11974

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• Ran make check-style to check for style errors (required for all pull requests)
• Ran make test to ensure unit tests passed
• Ran make flow to ensure type checking passed

[enahum]

Lgtm, but can we add a unit test and a perhaps add a comment of what is expected?

[sudheerDev]

@enahum Added a comment for now. I will remove the line when i address the ticket https://mattermost.atlassian.net/browse/MM-12169.

i don't see forcing everywhere to return an empty string as a good solution as we need someone only for user typing scenario. So, planning on changing to an empty string as default and someone if needed by passing param in displayUsername function

[enahum]

Ok make sure to change it everywhere, redux/mobile/webapp as needed once you do.

Feel free to merge this when ready

[sudheerDev]

@enahum Merging this.

Plan is to change all instances as most of them were changed by me. That's the plan.