Add `getOptionalEnvVar` helper #10707

Job	Run time
check-codeql-versions	35s
analyze-other (actions)	50s
analyze-other (python)	55s
analyze-javascript (ubuntu-22.04)	4m 36s
analyze-javascript (ubuntu-22.04, linked)	4m 0s
analyze-javascript (windows-2022)	6m 7s
analyze-javascript (windows-2022, linked)	5m 2s
analyze-javascript (ubuntu-24.04, linked)	4m 14s
analyze-javascript (ubuntu-24.04)	4m 14s
analyze-javascript (windows-2025)	6m 13s
analyze-javascript (macos-15)	8m 27s
analyze-javascript (macos-13, linked)	13m 23s
analyze-javascript (macos-13)	7m 19s
analyze-javascript (windows-2025, linked)	5m 27s
analyze-javascript (macos-14, linked)	10m 11s
analyze-javascript (macos-14)	6m 54s
analyze-javascript (macos-15, linked)	8m 55s
	1h 37m 22s

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