Fix client mTLS with default system trust

Author: Debashismitra01

exporters/sender/okhttp/src/main/java/io/opentelemetry/exporter/sender/okhttp/internal/OkHttpHttpSender.java

@@ -7,6 +7,8 @@
 
 import io.opentelemetry.api.impl.InstrumentationUtil;
 import io.opentelemetry.exporter.internal.RetryUtil;
+import io.opentelemetry.exporter.internal.TlsUtil;
+import javax.net.ssl.SSLException;
 import io.opentelemetry.sdk.common.CompletableResultCode;
 import io.opentelemetry.sdk.common.export.Compressor;
 import io.opentelemetry.sdk.common.export.HttpResponse;
@@ -108,8 +110,19 @@ public OkHttpHttpSender(
     boolean isPlainHttp = endpoint.getScheme().equals("http");
     if (isPlainHttp) {
       builder.connectionSpecs(Collections.singletonList(ConnectionSpec.CLEARTEXT));
-    } else if (sslContext != null && trustManager != null) {
-      builder.sslSocketFactory(sslContext.getSocketFactory(), trustManager);
+    } else if (sslContext != null) {
+      X509TrustManager effectiveTrustManager = trustManager;
+
+      if (effectiveTrustManager == null) {
+        try {
+          effectiveTrustManager = TlsUtil.defaultTrustManager();
+        } catch (SSLException e) {
+          throw new IllegalStateException("Unable to initialize default trust manager", e);
+        }
+      }
+      builder.sslSocketFactory(
+          sslContext.getSocketFactory(),
+          effectiveTrustManager);
     }
 
     this.client = builder.build();