Stops Integer boxing via null checks in Observable.concatEager

[jnlopar]

Replaces with verifyPositive as in other similar methods.

This seems to be a trivial change and makes this more consistent with other areas of the code. Let me know if it needs its own issue or any specific tests. Given the methods only accept primitive ints, and passing a null Integer would throw a runtime exception, I'm not sure there's anything needed outside of the already present testing.

[codecov]

Codecov Report

Merging #6012 into 2.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6012      +/-   ##
============================================
- Coverage     98.28%   98.26%   -0.03%     
+ Complexity     6161     6159       -2     
============================================
  Files           659      659              
  Lines         44522    44522              
  Branches       6201     6201              
============================================
- Hits          43757    43748       -9     
+ Misses          233      231       -2     
- Partials        532      543      +11

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	539 <2> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-5.89%)	2% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-2.39%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.91% <0%> (-2.09%)	56% <0%> (-2%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	90.29% <0%> (-1.5%)	2% <0%> (ø)
... and 23 more

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[artem-zinnatullin]

Noice

also means that there were no unit tests that'd check boundaries of the parameters but it's not that critical tbh

[akarnokd]

There is already a reflection based range check for the base classes so these null checks can be simply removed; the delegated calls should perform the verifyPositive calls already too.

[akarnokd]

Thanks for contribution. I've posted a broader solution to this problem in #6014.