Prevent generating test case with mask filled 0 only

modules/core/test/ocl/test_arithm.cpp

@@ -157,6 +157,7 @@ PARAM_TEST_CASE(ArithmTestBase, MatDepth, Channels, bool)
         Border maskBorder = randomBorder(0, use_roi ? MAX_VALUE : 0);
         randomSubMat(mask, mask_roi, roiSize, maskBorder, CV_8UC1, 0, 2);
         cv::threshold(mask, mask, 0.5, 255., CV_8UC1);
+        *mask.ptr(0) = 255; // prevent test case with mask filled 0 only
 
         val = cv::Scalar(rng.uniform(-100.0, 100.0), rng.uniform(-100.0, 100.0),
                          rng.uniform(-100.0, 100.0), rng.uniform(-100.0, 100.0));