fix: accept no streams in concatReadableStreams

[jrvidal]

I was surprised to find that this case is not contemplated. My gut feeling is that we should return an empty stream.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (e5c2055).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7173      +/-   ##
==========================================
- Coverage   94.57%   94.57%   -0.01%     
==========================================
  Files         636      636              
  Lines       52142    52145       +3     
  Branches     9401     9403       +2     
==========================================
+ Hits        49315    49317       +2     
- Misses       2249     2250       +1     
  Partials      578      578              

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