fix(chat/web): add early guard and use stable keys for reactions

[nitin-kumar-code]

Summary

Improves ChatMessage robustness and rendering behavior by addressing two issues.

Changes

• Added an early return guard to avoid rendering when message is undefined or invalid
• Replaced index-based keys with stable keys for reaction elements

Benefits

• Prevents potential runtime errors when message data is missing
• Avoids React rendering issues caused by unstable list keys

Notes

• No functional changes
• Minimal and focused improvements

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.