feat: Allow store reading value as null

[weichou1229]

Allow store reading value as null

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PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?)
• I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• I have opened a PR for the related docs change (if not, why?)

Testing Instructions

Test with simple device service.

New Dependency Instructions (If applicable)

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.52%. Comparing base (c49e7a4) to head (11f0ae7).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #933   +/-   ##
=======================================
  Coverage   43.52%   43.52%           
=======================================
  Files         124      124           
  Lines        6752     6752           
=======================================
  Hits         2939     2939           
  Misses       3557     3557           
  Partials      256      256           

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[cloudxxx8]

LGTM