fix: sending empty data in sse in js client

[euri10]

Description

• from our discussion here https://discord.com/channels/919193495116337154/1213242966592856164 it appears that javascript clients dont "see" an event that has no data, while we explicitly test for this and that httpx-sse receives the event fine in that case.
• the spec https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream is not that clear to me as to whether or not an event without data is ok.
• Considering the EventSource "client" is not ok with it, and that it's so easy DX-wise to make the mistake not explicitely sending it we fix it by defaulting to the empty-string

The added app is just to manually test the fix works, I dont know how we could incorporate it in our test suite given it involves running a server and checkin if the html page yields the event fine, maybe in e2e testing ?

[provinzkraut]

Just a thought, something like this would be good to test end-to-end with a headless browser. @euri10 wdyt?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.23%. Comparing base (1fb981d) to head (8e5682a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3176      +/-   ##
==========================================
- Coverage   98.23%   98.23%   -0.01%     
==========================================
  Files         320      320              
  Lines       14425    14431       +6     
  Branches     2322     2324       +2     
==========================================
+ Hits        14171    14176       +5     
  Misses        113      113              
- Partials      141      142       +1     

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[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/3176

[JacobCoffee]

@provinzkraut how would we do this?

[provinzkraut]

Probably with playwright or something.

[euri10]

I'm all for it but it seems sse is not supported: microsoft/playwright#7466
this is the best I came upwith and it sends error
see #3183