Implement Payload<String, Any?> changes

[zambrovski]

• fix As a user I want to pass payload to commands that contain null values #268

[zambrovski]

LGTM. Thank you @emaarco

[codecov]

Codecov Report

❌ Patch coverage is 26.66667% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.04%. Comparing base (f76f147) to head (3cdb1e2).
⚠️ Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
...pi/process/StartProcessByDefinitionAtElementCmd.kt	0.00%	2 Missing ⚠️
...ssengineapi/process/StartProcessByDefinitionCmd.kt	0.00%	2 Missing ⚠️
...ocessengineapi/process/StartProcessByMessageCmd.kt	0.00%	2 Missing ⚠️
...ters/processengineapi/correlation/SendSignalCmd.kt	0.00%	1 Missing ⚠️
...gineapi/decision/DecisionByRefEvaluationCommand.kt	0.00%	1 Missing ⚠️
...rs/processengineapi/task/CompleteTaskByErrorCmd.kt	0.00%	1 Missing ⚠️
...mcrafters/processengineapi/task/CompleteTaskCmd.kt	0.00%	1 Missing ⚠️
...rocessengineapi/task/support/AssignmentDetector.kt	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #274   +/-   ##
==========================================
  Coverage      46.04%   46.04%           
  Complexity        67       67           
==========================================
  Files             41       41           
  Lines            417      417           
  Branches           9        9           
==========================================
  Hits             192      192           
  Misses           220      220           
  Partials           5        5           

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