dnn: fix build (nullptr issue)

modules/dnn/src/layers/proposal_layer.cpp

@@ -287,7 +287,7 @@ public:
         CV_Assert(imInfo.total() >= 2);
         // We've chosen the smallest data type because we need just a shape from it.
         // We don't allocate memory but just need the shape is correct.
-        Mat fakeImageBlob(shape(1, 1, imInfo.at<float>(0), imInfo.at<float>(1)), CV_8UC1, nullptr);
+        Mat fakeImageBlob(shape(1, 1, imInfo.at<float>(0), imInfo.at<float>(1)), CV_8UC1, NULL);
 
         // Generate prior boxes.
         std::vector<Mat> layerInputs(2), layerOutputs(1, priorBoxes);