feat: Allow reading value to be null

[weichou1229]

Allow reading value to be 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 device simple.

New Dependency Instructions (If applicable)

[cloudxxx8]

LGTM

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 32.65%. Comparing base (1b8f5c3) to head (888d014).

Files with missing lines	Patch %	Lines
pkg/models/commandvalue.go	0.00%	3 Missing and 1 partial ⚠️
internal/transformer/transform.go	0.00%	1 Missing and 1 partial ⚠️
internal/transformer/transformresult.go	0.00%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1633      +/-   ##
==========================================
- Coverage   32.71%   32.65%   -0.06%     
==========================================
  Files          48       48              
  Lines        4738     4746       +8     
==========================================
  Hits         1550     1550              
- Misses       3052     3057       +5     
- Partials      136      139       +3     

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