Use NULL instead of nullptr to preserve the compatibility with cxx03

unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h b/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
index 359fd24..caf9949 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
@@ -811,7 +811,7 @@
   }
 
   EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE Scalar* data() const {
-    return nullptr;
+    return NULL;
   }
 
  protected: