MacOS - Certificate Generator Fails to Create Certificates

**Describe the bug**
Certificate Generator throws an exception during certificate installation on MacOS

**To Reproduce**
Steps to reproduce the behavior:
1. Run SelfHostedCoreWCFService / CertificateGenerator on MacOS. The code will throw an exception.
2. Full call stacks.
Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
at WcfTestCommon.CertificateGenerator.**CreateCertificate**(Boolean isAuthority, Boolean isMachineCert, X509Certificate signingCertificate, CertificateCreationSettings certificateCreationSettings) in /_/src/System.Private.ServiceModel/tools/CertificateGenerator/CertificateGeneratorLibrary/CertificateGenerator.cs:line 499
at WcfTestCommon.CertificateGenerator.CreateMachineCertificate(CertificateCreationSettings creationSettings) in /_/src/System.Private.ServiceModel/tools/CertificateGenerator/CertificateGeneratorLibrary/CertificateGenerator.cs:line 228
at WcfTestCommon.CertificateManager.CreateAndInstallLocalMachineCertificates(CertificateGenerator certificateGenerator) in /_/src/System.Private.ServiceModel/tools/CertificateGenerator/CertificateGeneratorLibrary/CertificateManager.cs:line 165
at CertificateGeneratorLibrary.SetupCerts(String testserverbase, TimeSpan validatePeriod, String crlFileLocation, Int32 httpPort) in /_/src/System.Private.ServiceModel/tools/CertificateGenerator/CertificateGeneratorLibrary/CertificateGeneratorLibrary.cs:line 76

**Expected behavior**
The code should generate certificates the same way it does on Windows/Linux.