Fix NPE happening after cancellation in MultiSelectFirstOp

[cescoffier]

Fix #605.
Keep a reference on the downstream so we can send the completion event after the cancellation

[codecov]

Codecov Report

Merging #606 (2ee239a) into main (729fb5a) will increase coverage by 0.13%.
The diff coverage is 66.66%.

❗ Current head 2ee239a differs from pull request most recent head 01a5d4c. Consider uploading reports for the commit 01a5d4c to get more accurate results

@@             Coverage Diff              @@
##               main     #606      +/-   ##
============================================
+ Coverage     90.00%   90.14%   +0.13%     
- Complexity     3005     3006       +1     
============================================
  Files           392      392              
  Lines         11697    11698       +1     
  Branches       1460     1462       +2     
============================================
+ Hits          10528    10545      +17     
+ Misses          601      590      -11     
+ Partials        568      563       -5     

Impacted Files	Coverage Δ
...rye/mutiny/operators/multi/MultiSelectFirstOp.java	90.00% <66.66%> (+10.51%)	⬆️
...iny/operators/multi/builders/BaseMultiEmitter.java	79.16% <0.00%> (-8.34%)	⬇️
...perators/multi/processors/SerializedProcessor.java	85.14% <0.00%> (-6.94%)	⬇️
...y/operators/multi/processors/UnicastProcessor.java	95.57% <0.00%> (-1.77%)	⬇️
...smallrye/mutiny/helpers/test/AssertSubscriber.java	90.22% <0.00%> (-0.76%)	⬇️
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	85.98% <0.00%> (-0.32%)	⬇️
...utiny/operators/multi/MultiWindowOnDurationOp.java	70.71% <0.00%> (+0.71%)	⬆️
...lrye/mutiny/subscription/SerializedSubscriber.java	88.67% <0.00%> (+0.94%)	⬆️
...smallrye/mutiny/operators/multi/MultiEmitOnOp.java	90.47% <0.00%> (+0.95%)	⬆️
...erators/multi/builders/SerializedMultiEmitter.java	82.27% <0.00%> (+1.26%)	⬆️
... and 11 more