Fix #2740: Use CpuInfo.Unknown if CpuDetector.Detect() does not find supported OS

[BraedenLS]

The nullability of HostEnvironmentInfo's Cpu property was mistakenly going unnoticed. This caused a null reference exception in the ZeroMeasurementAnalyser. I corrected this by making it nullable and checking for it in the analyser. Preventing it from breaking for all not-yet supported OS CpuDetectors

[BraedenLS]

@dotnet-policy-service agree company="Lake Shore Cryotronics"

[AndreyAkinshin]

I believe the main issue is that HostEnvironmentInfo assigns null to the non-nullable Cpu. I would prefer to keep HostEnvironmentInfo.Cpu as Lazy<CpuInfo> (a non-nullable value) to avoid multiple null checks. CpuInfo is already designed to potentially miss any of its properties. A better solution would be to introduce CpuInfo.Unknown on the Perfolizer side and use it in HostEnvironmentInfo if all the CpuDetectors fail to detect the CPU. I will release a new version of Perfolizer and bump it in BenchmarkDotNet.

[AndreyAkinshin]

CpuInfo.Unknown is available in the latest master. @BraedenLS, I suggest upgrading the HostEnvironmentInfo.Cpu lazy definition to use CpuInfo.Unknown if CpuDetector.CrossPlatform.Detect() returns null.

[BraedenLS]

Rebased and implemented.