Fix fast synch when fence is waited before a command buffer is created (#2657)

mlx/backend/metal/device.cpp

@@ -471,6 +471,10 @@ void Device::end_encoding(int index) {
 CommandEncoder& Device::get_command_encoder(int index) {
   auto& stream = get_stream_(index);
   if (stream.encoder == nullptr) {
+    // Ensure there is an active command buffer
+    if (stream.buffer == nullptr) {
+      get_command_buffer(index);
+    }
     stream.encoder = std::make_unique<CommandEncoder>(stream);
     stream.fence = std::make_shared<Fence>(device_->newFence());
   }