allow null constant_value in Pad and ignore it when loading

modules/dnn/src/onnx/onnx_importer.cpp

@@ -2605,7 +2605,8 @@ void ONNXImporter::parsePad(LayerParams& layerParams, const opencv_onnx::NodePro
         paddings = paddings.t();
         layerParams.set("paddings", DictValue::arrayInt(paddings.ptr<int>(), paddings.total()));
 
-        if (node_proto.input_size() == 3)
+        // check for non-null constant_value
+        if (node_proto.input_size() == 3 && !node_proto.input(2).empty())
         {
             Mat value = getBlob(node_proto, 2);
             float padValue = (depth == CV_8S) ? (float)value.ptr<int8_t>()[0] : value.ptr<float>()[0];