map reduce test

Hi @noctarius,  while making some stabilizer test for map reduce,  I found some unexpected results.

In this test, the size of the result data structure,  is not matching the size of the expected results.  there seems to be lots of empty structures included.

a sample visual from this test when printing to std out 
```
{68=[], 0=[], 70=[], 2=[], 64=[], 4=[], 66=[], 6=[], 8=[], 76=[], 10=[], 78=[], 72=[], 12=[Employee{id=12, name='bbb', age=11, active=false, salary=925.7860162655732}], 74=[], 14=[Employee{id=14, name='ccc', age=35, active=true, salary=323.7819968806972}], 84=[], 16=[Employee{id=16, name='aaa', age=6, active=true, salary=639.5925632250688}], 18=[Employee{id=18, name='bbb', age=34, active=false, salary=685.7523578895596}], 86=[], 80=[], 20=[], 22=[Employee{id=22, name='ddd', age=9, active=false, salary=245.88783556064863}], 82=[], 92=[], 24=[Employee{id=24, name='ggg', age=9, active=true, salary=26.618775576506938}], 26=[Employee{id=26, name='ccc', age=8, active=false, salary=94.58498366900325}], 94=[], 28=[Employee{id=28, name='ggg', age=24, active=false, salary=478.6757707844015}], 88=[], 90=[], 30=[], 34=[], 32=[], 98=[], 38=[], 96=[], 36=[], 42=[], 40=[], 46=[], 44=[], 50=[], 48=[], 54=[], 52=[], 58=[], 56=[], 62=[], 60=[]}
50
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[Employee{id=12, name='bbb', age=11, active=false, salary=925.7860162655732}]
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[Employee{id=14, name='ccc', age=35, active=true, salary=323.7819968806972}]
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[Employee{id=16, name='aaa', age=6, active=true, salary=639.5925632250688}]
[Employee{id=18, name='bbb', age=34, active=false, salary=685.7523578895596}]
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[Employee{id=22, name='ddd', age=9, active=false, salary=245.88783556064863}]
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[Employee{id=24, name='ggg', age=9, active=true, salary=26.618775576506938}]
[Employee{id=26, name='ccc', age=8, active=false, salary=94.58498366900325}]
[]
[Employee{id=28, name='ggg', age=24, active=false, salary=478.6757707844015}]
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```