Matter Thermostat: check for nil in temp attribute handler

[ctowns]

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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/5z2RL48BA7j1

[github-actions]

Test Results

   64 files  ±0    402 suites  ±0   0s ⏱️ ±0s
2 000 tests ±0  2 000 ✅ ±0  0 💤 ±0  0 ❌ ±0 
3 452 runs  ±0  3 452 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit f44090f. ± Comparison against base commit f150282.

[github-actions]

File	Coverage
All files	83%	❌
/home/runner/work/SmartThingsEdgeDrivers/SmartThingsEdgeDrivers/drivers/SmartThings/matter-thermostat/src/init.lua	85%	❌
/home/runner/work/SmartThingsEdgeDrivers/SmartThingsEdgeDrivers/drivers/SmartThings/matter-thermostat/src/embedded-cluster-utils.lua	42%	❌

Minimum allowed coverage is 90%

Generated by 🐒 cobertura-action against f44090f