Matter Sensor: return nil if no unit conversion is possible

[ctowns]

Check all that apply

Type of Change

• [ ] WWST Certification Request
  • If this is your first time contributing code:
    • [ ] I have reviewed the README.md file
    • [ ] I have reviewed the CODE_OF_CONDUCT.md file
    • [ ] I have signed the CLA
  • [ ] I plan on entering a WWST Certification Request or have entered a request through the WWST Certification console at developer.smartthings.com
• [x] Bug fix
• [ ] New feature
• [ ] Refactor

Checklist

• [ ] I have performed a self-review of my code
• [ ] I have commented my code in hard-to-understand areas
• [ ] I have verified my changes by testing with a device or have communicated a plan for testing
• [ ] I am adding new behavior, such as adding a sub-driver, and have added and run new unit tests to cover the new behavior

Description of Change

Summary of Completed Tests

[github-actions[bot]]

Invitation URL: https://bestow-regional.api.smartthings.com/invite/Boj0zD73vn2A

[github-actions[bot]]

File	Coverage
All files	87%	:x:
/home/runner/work/SmartThingsEdgeDrivers/SmartThingsEdgeDrivers/drivers/SmartThings/matter-sensor/src/smoke-co-alarm/init.lua	83%	:x:
/home/runner/work/SmartThingsEdgeDrivers/SmartThingsEdgeDrivers/drivers/SmartThings/matter-sensor/src/init.lua	91%	:white_check_mark:
/home/runner/work/SmartThingsEdgeDrivers/SmartThingsEdgeDrivers/drivers/SmartThings/matter-sensor/src/air-quality-sensor/init.lua	92%	:white_check_mark:
/home/runner/work/SmartThingsEdgeDrivers/SmartThingsEdgeDrivers/drivers/SmartThings/matter-sensor/src/embedded-cluster-utils.lua	42%	:x:

Minimum allowed coverage is 90%

Generated by :monkey: cobertura-action against 602bced7425981a5551e35879da067bf0545b606

[github-actions[bot]]

Test Results

   61 files  ±0    375 suites  ±0   0s :stopwatch: ±0s 1 822 tests +1  1 822 :white_check_mark: +1  0 :zzz: ±0  0 :x: ±0  3 170 runs  +1  3 170 :white_check_mark: +1  0 :zzz: ±0  0 :x: ±0 

Results for commit 602bced7. ± Comparison against base commit 2125c846.

:recycle: This comment has been updated with latest results.

[nickolas-deboom]

LGTM!