fix: concatMap early first upstream issues

[jponge]

This fixes the NPE #1498 and reveals a deadlock problem.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (32fdd3e) 89.33% compared to head (f18296b) 89.39%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1499      +/-   ##
============================================
+ Coverage     89.33%   89.39%   +0.06%     
+ Complexity     3368     3367       -1     
============================================
  Files           459      459              
  Lines         13435    13437       +2     
  Branches       1636     1637       +1     
============================================
+ Hits          12002    12012      +10     
- Misses          796      803       +7     
+ Partials        637      622      -15     

Files	Coverage Δ
...llrye/mutiny/operators/multi/MultiConcatMapOp.java	88.88% <90.00%> (+1.47%)	⬆️

... and 12 files with indirect coverage changes