feat: Using isNull field instead of nil reading value

[weichou1229]

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• 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?)
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Testing Instructions

• make test
• test with simple DS

New Dependency Instructions (If applicable)

[weichou1229]

This PR is based on the core-contracts changes edgexfoundry/go-mod-core-contracts#935

[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

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 41.17647% with 10 lines in your changes missing coverage. Please review.

Project coverage is 31.81%. Comparing base (0f2c1aa) to head (f2bf259).

Files with missing lines	Patch %	Lines
internal/pkg/infrastructure/redis/reading.go	46.66%	7 Missing and 1 partial ⚠️
internal/pkg/infrastructure/postgres/reading.go	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4974   +/-   ##
=======================================
  Coverage   31.80%   31.81%           
=======================================
  Files         265      265           
  Lines       19470    19484   +14     
=======================================
+ Hits         6193     6199    +6     
- Misses      11854    11861    +7     
- Partials     1423     1424    +1     

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[cloudxxx8]

LGTM