2.x: Fix Flowable + Single elementAt and elementAtOrError operators on empty sources

[vanniktech]

This will fix #4680

• elementAtOrError(1) on an empty source just leaves the new Single hanging without signalling onError()
• elementAt(1) on an empty source just leaves the Maybe hanging without signalling onComplete()
• I also noticed that elementAt() with a default value on an empty source did not work either.

[codecov-io]

Current coverage is 82.15% (diff: 0.00%)

Merging #4681 into 2.x will increase coverage by 0.17%

@@                2.x      #4681   diff @@
==========================================
  Files           565        565          
  Lines         37426      37426          
  Methods           0          0          
  Messages          0          0          
  Branches       5746       5746          
==========================================
+ Hits          30682      30749    +67   
+ Misses         4666       4608    -58   
+ Partials       2078       2069     -9   

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[akarnokd]

You may have to check the build and restart it a few times if it fails with jacocoTestReport.