fix: timing issue in MultiOnSubscribeCall where the Multi could terminate before the Uni

[jponge]

Fixes #1678

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.44%. Comparing base (97f75a4) to head (9591080).
Report is 229 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/mutiny/operators/multi/MultiOnSubscribeCall.java	92.30%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1693      +/-   ##
============================================
+ Coverage     89.41%   89.44%   +0.02%     
+ Complexity     3275     3271       -4     
============================================
  Files           457      457              
  Lines         13264    13301      +37     
  Branches       1642     1649       +7     
============================================
+ Hits          11860    11897      +37     
- Misses          788      789       +1     
+ Partials        616      615       -1     

Files with missing lines	Coverage Δ
...e/mutiny/operators/multi/MultiOnSubscribeCall.java	87.71% <92.30%> (+4.38%)	⬆️

... and 11 files with indirect coverage changes