Do not include placeholder in serialize and properly replace placeholder for streams

[ahuang11]

Before, the placeholder disappeared, but no message.

Screen.Recording.2024-04-15.at.3.38.59.PM.mov

After fixes:

Screen.Recording.2024-04-15.at.3.39.32.PM.mov

[codecov]

Codecov Report

Attention: Patch coverage is 9.09091% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 37.00%. Comparing base (76c911a) to head (9b4f8ed).
Report is 8 commits behind head on main.

❗ Current head 9b4f8ed differs from pull request most recent head 068b479. Consider uploading reports for the commit 068b479 to get more accurate results

Files	Patch %	Lines
panel/tests/chat/test_feed.py	10.00%	18 Missing ⚠️
panel/chat/feed.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6734      +/-   ##
==========================================
- Coverage   40.01%   37.00%   -3.02%     
==========================================
  Files         313      305       -8     
  Lines       46194    46052     -142     
==========================================
- Hits        18486    17040    -1446     
- Misses      27708    29012    +1304     

Flag	Coverage Δ
ui-tests	37.00% <9.09%> (-3.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.