Fix stream None base message

[ahuang11]

This didn't stream anything before (and was actually stuck in an infinite while loop)

import panel as pn
chat_feed = pn.chat.ChatFeed()
message = pn.chat.ChatMessage(user="System")
chat_feed.send(message)
for i in range(0, 12):
    chat_feed.stream(f"new message {i}", message=message)

Originally discovered here:
https://discourse.holoviz.org/t/stream-text-to-chatinterface/6976/15?u=ahuang11

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.91%. Comparing base (864a514) to head (a46858b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6522      +/-   ##
==========================================
- Coverage   82.91%   82.91%   -0.01%     
==========================================
  Files         313      313              
  Lines       45902    45917      +15     
==========================================
+ Hits        38060    38070      +10     
- Misses       7842     7847       +5     

Flag	Coverage Δ
ui-tests	40.10% <18.18%> (-0.01%)	⬇️
unitexamples-tests	71.13% <100.00%> (-0.01%)	⬇️

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