feat: Allow store reading as null value

[weichou1229]

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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
• Run make test

New Dependency Instructions (If applicable)

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[cloudxxx8]

LGTM

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 31.81%. Comparing base (eff0073) to head (faff7e1).

Files with missing lines	Patch %	Lines
internal/pkg/infrastructure/postgres/reading.go	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4950   +/-   ##
=======================================
  Coverage   31.81%   31.81%           
=======================================
  Files         265      265           
  Lines       19338    19338           
=======================================
  Hits         6152     6152           
  Misses      11792    11792           
  Partials     1394     1394           

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