_last_known_duration(): add extra guarding #1772

Job	Run time
Shellcheck	5s
Prepare	53s
Dependency	4s
Ruff check and force	6s
Check commit	47s
Create pytest cache for Python 3.13	6s
Run pytest using Python 3.13	24s
Run mypy	6s
Process test coverage	11s
Build and publish Python 🐍 distributions 📦 to TestPyPI	21s
Process test complexity	9s
	3m 12s

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