Remove duplicate nullity check line from Observable.toMap()

[philleonard]

Small cleanup to remove duplicate nullity check line from Observable#toMap

[codecov]

Codecov Report

Merging #5723 into 2.x will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5723      +/-   ##
============================================
- Coverage     96.33%   96.27%   -0.07%     
+ Complexity     5822     5820       -2     
============================================
  Files           634      634              
  Lines         41604    41603       -1     
  Branches       5761     5761              
============================================
- Hits          40079    40052      -27     
- Misses          599      619      +20     
- Partials        926      932       +6

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	508 <0> (ø)	⬇️
...in/java/io/reactivex/subjects/BehaviorSubject.java	85.93% <0%> (-6.78%)	57% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	90.26% <0%> (-4.87%)	61% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	93.83% <0%> (-1.77%)	11% <0%> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	93.44% <0%> (-1.64%)	2% <0%> (ø)
...vex/internal/subscribers/QueueDrainSubscriber.java	50% <0%> (-1.57%)	15% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	90.29% <0%> (-1.5%)	2% <0%> (ø)
... and 24 more

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