Apply code review feedback to use more appropriate helper functions in parsing a boolean config option

Author: Michael J. Lyons (XBOX)

src/shared/Microsoft.AzureRepos/AzureReposHostProvider.cs

@@ -549,13 +549,11 @@ private bool UseServicePrincipal(out ServicePrincipalIdentity sp)
 
             if (hasCertThumbprint)
             {
-                bool hasX5CSetting = _context.Settings.TryGetSetting(
+                sp.SendX5C = _context.Settings.TryGetSetting(
                     AzureDevOpsConstants.EnvironmentVariables.ServicePrincipalCertificateSendX5C,
                     Constants.GitConfiguration.Credential.SectionName,
                     AzureDevOpsConstants.GitConfiguration.Credential.ServicePrincipalCertificateSendX5C,
-                    out string certHasX5C);
-
-                sp.SendX5C = !hasX5CSetting || certHasX5C != "false";
+                    out string certHasX5CStr) && certHasX5CStr.ToBooleanyOrDefault(false);
 
                 X509Certificate2 cert = X509Utils.GetCertificateByThumbprint(certThumbprint);
                 if (cert is null)