fix(http): handle empty values and multi-line comments in ServerSentEventStream

[tomas-zijdemans]

Bug 1: Empty values silently dropped. Fields like { data: "" } or { id: 0 } produced no output because the code used truthy checks (if (message.data)), which treat "" and 0 as false. Fixed by checking !== undefined instead.

Bug 2: Multi-line comments threw an error. The serializer rejected comments containing newlines with a SyntaxError, but comments are per-line constructs in SSE. This already worked correctly for data, just not for comments. Fixed by splitting on newlines instead of throwing.

See ServerSentEventParseStream for a roundtrip example (that requires this fix to work correctly)

Note: id?: string | number; in the ServerSentEventMessageinterface is a spec violation (it should only be string). I didn't change it, since that would be a breaking change. @kt3k: please let me know if you'd like me to include that as well.

Spec: https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.36%. Comparing base (6df7f21) to head (0a5d68d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7051      +/-   ##
==========================================
+ Coverage   94.01%   94.36%   +0.34%     
==========================================
  Files         627      627              
  Lines       50029    50031       +2     
  Branches     8807     8808       +1     
==========================================
+ Hits        47037    47212     +175     
+ Misses       2413     2251     -162     
+ Partials      579      568      -11     

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