Fix handling shell command when no argument is given.

```
adb shell cmd virtual_camera
```

Bug: 301023410
Test: atest virtual_camera_tests
Change-Id: Id7759525a2e1bbf1df7e53e4adbb287d60f73326
diff --git a/services/camera/virtualcamera/VirtualCameraService.cc b/services/camera/virtualcamera/VirtualCameraService.cc
index a2054da..08bfff7 100644
--- a/services/camera/virtualcamera/VirtualCameraService.cc
+++ b/services/camera/virtualcamera/VirtualCameraService.cc
@@ -185,6 +185,8 @@
                                                          uint32_t numArgs) {
   if (numArgs <= 0) {
     dprintf(out, kShellCmdHelp);
+    fsync(out);
+    return STATUS_OK;
   }
 
   if (args == nullptr || args[0] == nullptr) {