Merge pull request #25130 from CSBVision:patch-6

Update net_impl.cpp and net.cpp to fix the first part of #25112

modules/dnn/src/net.cpp

@@ -13,6 +13,7 @@ CV__DNN_INLINE_NS_BEGIN
 Net::Net()
     : impl(makePtr<Net::Impl>())
 {
+    setPreferableBackend(DNN_BACKEND_DEFAULT);
 }
 
 Net::~Net()