2.x: Fix & prevent null checks on primitives

[akarnokd]

This PR removes the accidental null checks on primitives in two Observable methods and adds a trap method to ObjectHelper to both highlight and fail the tests in case the null checks are still attempted. The method is marked as deprecated so it will also show up in (Eclipse's) problems window.

Replaces #6012
Resolves #6013

[codecov]

Codecov Report

Merging #6014 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x   #6014      +/-   ##
===========================================
+ Coverage     98.28%   98.3%   +0.02%     
- Complexity     6161    6162       +1     
===========================================
  Files           659     659              
  Lines         44522   44519       -3     
  Branches       6201    6201              
===========================================
+ Hits          43757   43763       +6     
+ Misses          233     227       -6     
+ Partials        532     529       -3

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	539 <0> (ø)	⬇️
.../io/reactivex/internal/functions/ObjectHelper.java	100% <100%> (ø)	22 <1> (+1)	⬆️
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.68% <0%> (-1.94%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-1.31%)	2% <0%> (ø)
... and 20 more

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[artem-zinnatullin]

Probably not the best choice of error type, InternalError indicates JVM error, combined with "Null check on a primitive" message might mislead the developer into thinking that something is wrong with bytecode/VM hehe

[akarnokd]

Especially if they run a value types enabled vm ;)