chore: add checks to avoid panics if the users return null

[yhl25]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 18 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@a884673). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
.../java/io/numaproj/numaflow/mapper/MapperActor.java	0.00%	4 Missing and 1 partial ⚠️
...j/numaflow/sourcetransformer/TransformerActor.java	0.00%	4 Missing and 1 partial ⚠️
...java/io/numaproj/numaflow/batchmapper/Service.java	80.00%	0 Missing and 4 partials ⚠️
...aproj/numaflow/mapstreamer/OutputObserverImpl.java	0.00%	1 Missing and 1 partial ⚠️
...main/java/io/numaproj/numaflow/sinker/Service.java	83.33%	0 Missing and 1 partial ⚠️
...io/numaproj/numaflow/sourcetransformer/Server.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #180   +/-   ##
=======================================
  Coverage        ?   59.73%           
  Complexity      ?      467           
=======================================
  Files           ?      151           
  Lines           ?     3368           
  Branches        ?      205           
=======================================
  Hits            ?     2012           
  Misses          ?     1213           
  Partials        ?      143           

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