Ignore nil channels in Discard

[destel]

The behavior is now consistent with other non-blocking stages.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.76%. Comparing base (3a2042d) to head (e22e5aa).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #107   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files          15       15           
  Lines         835      837    +2     
=======================================
+ Hits          833      835    +2     
  Misses          2        2           

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