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:

• [x] I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• [x] I am not introducing a new dependency (add notes below if you are)
• [x] I have added unit tests for the new feature or bug fix (if not, why?)
• [x] 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 :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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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