use 'empty' method to check for emptiness instead of 'length'

Author: DIlkhush00

io/src/openni2/openni2_device.cpp

@@ -56,7 +56,7 @@ pcl::io::openni2::OpenNI2Device::OpenNI2Device (const std::string& device_URI)
 
   openni_device_.reset (new openni::Device);
 
-  if (device_URI.length () > 0)
+  if (!device_URI.empty())
     status = openni_device_->open (device_URI.c_str ());
   else
     status = openni_device_->open (openni::ANY_DEVICE);