Merge pull request #25945 from vrabaud:02_fix

Fix size() for 0d matrix #25945

### Pull Request Readiness Checklist

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- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/core/include/opencv2/core/mat.inl.hpp

@@ -1275,7 +1275,7 @@ Size MatSize::operator()() const
 {
     int d = dims();
     CV_DbgAssert(d <= 2);
-    return d == 2 ? Size(p[1], p[0]) : Size(p[0], 1);
+    return d == 2 ? Size(p[1], p[0]) : d == 1 ? Size(p[0], 1) : Size(0, 0);
 }
 
 inline

modules/core/include/opencv2/core/types.hpp

@@ -320,10 +320,13 @@ struct Type< Point3_<_Tp> > { enum { value = CV_MAKETYPE(Depth<_Tp>::value, 3) }
 
 /** @brief Template class for specifying the size of an image or rectangle.
 
-The class includes two members called width and height. The structure can be converted to and from
-the old OpenCV structures CvSize and CvSize2D32f . The same set of arithmetic and comparison
+The class includes two members called width and height. The same set of arithmetic and comparison
 operations as for Point_ is available.
 
+For a 1d matrix, the size is (width, 1) and for a 0d matrix, it is (1, 1).
+
+For an empty matrix, it is (0, 0).
+
 OpenCV defines the following Size_\<\> aliases:
 @code
     typedef Size_<int> Size2i;

modules/core/test/test_mat.cpp

@@ -2414,7 +2414,15 @@ TEST(Mat, regression_18473)
     EXPECT_EQ((int)5, (int)m.at<short>(19, 49, 99));
 }
 
-// FITIT: remove DISABLE_ when 1D Mat is supported
+TEST(Mat0D, basic)
+{
+    Mat1b m1, m2(0, nullptr);
+    ASSERT_EQ(0, m1.size().width);
+    ASSERT_EQ(0, m1.size().height);
+    ASSERT_EQ(1, m2.size().width);
+    ASSERT_EQ(1, m2.size().height);
+}
+
 TEST(Mat1D, basic)
 {
     std::vector<int> sizes { 100 };