Use std::optional if available. #2028

Job	Run time
PSV.Linux.22.04.gcc9.Tests.CodeCov	10m 36s
PSV.MacOS15.Xcode16	2m 42s
PSV.Linux.22.04.gcc13	8m 32s
PSV.Linux.22.04.gcc11	8m 11s
PSV.iOS.MacOS13.Xcode14	13m 5s
PSV.Linux.22.04.gcc9.Cpplint	8m 46s
PSV.Linux.22.04.gcc9.OLP_SDK_ENABLE_DEFAULT_CACHE=OFF	3m 3s
PSV.Linux.22.04.gcc11.OLP_SDK_ENABLE_DEFAULT_CACHE=OFF	3m 5s
PSV.MacOS13.Xcode15.x86_64	4m 4s
PSV.Linux.22.04.clang	9m 7s
PSV.MacOS14.Xcode15	3m 50s
PSV.Linux.Android.22.04	6m 31s
PSV.Linux.latest.gcc14.OLP_SDK_ENABLE_DEFAULT_CACHE=OFF	2m 26s
PSV.iOS.MacOS13.Xcode15	17m 7s
PSV.iOS.MacOS14.Xcode15	5m 35s
PSV.iOS.MacOS15.Xcode16	5m 48s
PSV.Win.VC2022	10m 10s
PSV.Clang.Format.Checker	35s
PSV.Commit.Checker	6s
	2h 3m 19s

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