fix(): support new grpc client

Author: sandy2008

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExporterOptions.cs

@@ -273,6 +273,7 @@ internal HttpClient AddCertificatesToHttpClient(HttpClientHandler handler)
                     chain.ChainPolicy.CustomTrustStore.Add(trustedCertificate);
                     return chain.Build(cert);
                 }
+
                 return false;
             };
         }

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExporterOptionsExtensions.cs

@@ -42,7 +42,7 @@ public static Channel CreateChannel(this OtlpExporterOptions options)
                 string clientKeyPem = System.IO.File.ReadAllText(options.ClientKeyFile);
                 var keyPair = new KeyCertificatePair(clientCertPem, clientKeyPem);
 
-                string? rootCertPem = null;
+                string rootCertPem = string.Empty;
                 if (!string.IsNullOrEmpty(options.CertificateFile))
                 {
                     rootCertPem = System.IO.File.ReadAllText(options.CertificateFile);
@@ -52,11 +52,12 @@ public static Channel CreateChannel(this OtlpExporterOptions options)
             }
             else
             {
-                string? rootCertPem = null;
+                string rootCertPem = string.Empty;
                 if (!string.IsNullOrEmpty(options.CertificateFile))
                 {
                     rootCertPem = System.IO.File.ReadAllText(options.CertificateFile);
                 }
+
                 channelCredentials = new SslCredentials(rootCertPem);
             }
         }