fix: avoid NullPointerExceptions if array element is null

[adarsh0728]

fixes #193

[codecov]

Codecov Report

Attention: Patch coverage is 61.11111% with 7 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@da032c6). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...numaflow/accumulator/OutputStreamObserverImpl.java	0.00%	0 Missing and 1 partial ⚠️
...ava/io/numaproj/numaflow/mapper/MapperTestKit.java	0.00%	1 Missing ⚠️
...java/io/numaproj/numaflow/reducer/ReduceActor.java	50.00%	0 Missing and 1 partial ⚠️
...aflow/reducestreamer/OutputStreamObserverImpl.java	0.00%	0 Missing and 1 partial ⚠️
...aflow/sessionreducer/OutputStreamObserverImpl.java	0.00%	0 Missing and 1 partial ⚠️
...ava/io/numaproj/numaflow/sinker/SinkerTestKit.java	0.00%	1 Missing ⚠️
...ow/sourcetransformer/SourceTransformerTestKit.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #194   +/-   ##
=======================================
  Coverage        ?   58.80%           
  Complexity      ?      476           
=======================================
  Files           ?      151           
  Lines           ?     3389           
  Branches        ?      231           
=======================================
  Hits            ?     1993           
  Misses          ?     1229           
  Partials        ?      167           

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[adarsh0728]

Need to decide whether we want to preserve the null element in keys/tags