2.x: Fix Flowable.singleOrError().toFlowable() not signalling NoSuchElementException

[akarnokd]

When a singleOrError is followed by toFlowable(), the assembly process switches the Flowable->Single operator into a Flowable->Flowable operator implementing the singleOrError behavior (saving the back-and-forth type conversion). The backing implementation was shared with singleElement and as such, did not properly handle the orError case for an empty source. The PR fixes the lack of NoSuchElementException in this case.

Fixes #5903.

The Observable variant doesn't have such optimization but the test has been converted to make sure Observable is verified for this aspect in case the optimization is implemented with it in the future.

[codecov]

Codecov Report

Merging #5904 into 2.x will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5904      +/-   ##
============================================
+ Coverage     97.92%   97.97%   +0.04%     
- Complexity     5987     5989       +2     
============================================
  Files           655      655              
  Lines         43925    43929       +4     
  Branches       6087     6088       +1     
============================================
+ Hits          43014    43039      +25     
+ Misses          281      271      -10     
+ Partials        630      619      -11

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/flowable/FlowableSingleSingle.java	97.95% <100%> (ø)	3 <1> (ø)	⬇️
...ex/internal/operators/flowable/FlowableSingle.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...ternal/operators/flowable/FlowableSingleMaybe.java	100% <100%> (ø)	3 <1> (ø)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	95.65% <0%> (-2.61%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...internal/operators/observable/ObservableCache.java	93.7% <0%> (-1.58%)	9% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	86.58% <0%> (-1.28%)	3% <0%> (ø)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
... and 24 more

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