Pass NULL to grpc_channel_create_call for nil serverName

src/objective-c/GRPCClient/private/GRPCChannel.m

@@ -198,15 +198,20 @@ static grpc_channel_args *BuildChannelArgs(NSDictionary *dictionary) {
 - (grpc_call *)unmanagedCallWithPath:(NSString *)path
                           serverName:(NSString *)serverName
                      completionQueue:(GRPCCompletionQueue *)queue {
-  grpc_slice host_slice = grpc_slice_from_copied_string(serverName.UTF8String);
+  grpc_slice host_slice;
+  if (serverName) {
+    host_slice = grpc_slice_from_copied_string(serverName.UTF8String);
+  }
   grpc_slice path_slice = grpc_slice_from_copied_string(path.UTF8String);
   grpc_call *call = grpc_channel_create_call(_unmanagedChannel,
                                              NULL, GRPC_PROPAGATE_DEFAULTS,
                                              queue.unmanagedQueue,
                                              path_slice,
-                                             &host_slice,
+                                             serverName ? &host_slice : NULL,
                                              gpr_inf_future(GPR_CLOCK_REALTIME), NULL);
-  grpc_slice_unref(host_slice);
+  if (serverName) {
+    grpc_slice_unref(host_slice);
+  }
   grpc_slice_unref(path_slice);
   return call;
 }

src/objective-c/GRPCClient/private/GRPCHost.m

@@ -145,8 +145,7 @@ static GRPCConnectivityMonitor *connectivityMonitor = nil;
     }
     channel = _channel;
   }
-  NSString *name = serverName ? serverName : _address;
+  return [channel unmanagedCallWithPath:path serverName:serverName completionQueue:queue];
-  return [channel unmanagedCallWithPath:path serverName:name completionQueue:queue];
 }
 
 - (BOOL)setTLSPEMRootCerts:(nullable NSString *)pemRootCerts