Fix repetition when the first emitted item is null

[cescoffier]

Fix #447


[codecov]

Codecov Report

Merging #454 (14c4e5e) into master (77b3c3a) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #454      +/-   ##
============================================
+ Coverage     90.11%   90.18%   +0.06%     
- Complexity     2777     2779       +2     
============================================
  Files           373      373              
  Lines         10668    10669       +1     
  Branches       1309     1311       +2     
============================================
+ Hits           9614     9622       +8     
  Misses          559      559              
+ Partials        495      488       -7     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/smallrye/mutiny/groups/UniRepeat.java	100.00% <ø> (ø)	5.00 <0.00> (ø)
...rye/mutiny/operators/multi/MultiRepeatUntilOp.java	100.00% <100.00%> (ø)	5.00 <0.00> (ø)
.../io/smallrye/mutiny/helpers/queues/DrainUtils.java	86.66% <0.00%> (-8.89%)	13.00% <0.00%> (-3.00%)
...iny/operators/multi/builders/BaseMultiEmitter.java	83.33% <0.00%> (-4.17%)	20.00% <0.00%> (-1.00%)
...operators/multi/builders/CollectionBasedMulti.java	90.16% <0.00%> (-3.28%)	4.00% <0.00%> (ø%)
...erators/multi/builders/SerializedMultiEmitter.java	83.54% <0.00%> (-1.27%)	27.00% <0.00%> (+1.00%)	⬇️
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	86.91% <0.00%> (+0.62%)	3.00% <0.00%> (ø%)
...smallrye/mutiny/operators/multi/MultiEmitOnOp.java	90.47% <0.00%> (+0.95%)	2.00% <0.00%> (ø%)
...a/io/smallrye/mutiny/helpers/BlockingIterable.java	89.79% <0.00%> (+1.02%)	10.00% <0.00%> (ø%)
...erators/multi/builders/BufferItemMultiEmitter.java	90.00% <0.00%> (+1.42%)	25.00% <0.00%> (+1.00%)
... and 5 more