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
• 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]

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

[github-actions]

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

Minimum allowed coverage is 90%

Generated by 🐒 cobertura-action against 602bced

[github-actions]

Test Results

   61 files  ±0    375 suites  ±0   0s ⏱️ ±0s
1 822 tests +1  1 822 ✅ +1  0 💤 ±0  0 ❌ ±0 
3 170 runs  +1  3 170 ✅ +1  0 💤 ±0  0 ❌ ±0 

Results for commit 602bced. ± Comparison against base commit 2125c84.

♻️ This comment has been updated with latest results.

[nickolas-deboom]

LGTM!