fix: Remove JSON omitempty from SimpleReading value

[weichou1229]

The SimpleReading can contain the empty string.

Close #725

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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?) not impact the test
• 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?) not impact doc

Testing Instructions

• Run edgex and device service
• Query empty string value from device service
• Inspect the reading should contain empty string value

New Dependency Instructions (If applicable)

[cloudxxx8]

LGTM

[codecov-commenter]

Codecov Report

Merging #726 (d83bf3a) into main (ca0d31f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #726   +/-   ##
=======================================
  Coverage   63.79%   63.79%           
=======================================
  Files          79       79           
  Lines        3002     3002           
=======================================
  Hits         1915     1915           
  Misses        901      901           
  Partials      186      186           

Impacted Files	Coverage Δ
dtos/reading.go	74.64% <ø> (ø)

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[lenny-goodell]

LGTM