Make sure Truco is launched on non-empty and CV_8UC1 only.

This is checked in the function:
https://github.com/opencv/opencv/blob/3bb68212dac2e1c42e7910de9daf3a0766eaead8/modules/imgproc/src/contours_truco.cpp#L621

modules/imgproc/src/contours_new.cpp

@@ -649,7 +649,7 @@ void cv::findContours(InputArray _image,
     }
 
     // Fast path: RETR_LIST without hierarchy → findTRUContours (parallel contour extraction)
-    if (mode == RETR_LIST && !_hierarchy.needed())
+    if (mode == RETR_LIST && !_hierarchy.needed() && _image.type() == CV_8UC1)
     {
         // findTRUContours requires FOREGROUND=255; binarize=true thresholds the padded
         // image in-place, avoiding an extra allocation (findContours accepts any non-zero value)

modules/imgproc/src/contours_truco.cpp

@@ -618,7 +618,7 @@ void findTRUContours(InputArray _src, OutputArrayOfArrays _contours, int minSize
 {
     CV_INSTRUMENT_REGION();
     Mat src = _src.getMat();
-    CV_Assert(!src.empty() && src.type() == CV_8UC1);
+    CV_Assert(src.type() == CV_8UC1);
 
     // Buffer handling
     cv::Mat padded;