Merge pull request #26444 from dkurt:fix_empty_mat_assert

Fix empty mat debug assertion #26444

### 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/src/persistence_types.cpp

@@ -152,7 +152,10 @@ void read(const FileNode& node, Mat& m, const Mat& default_mat)
 
     size_t nelems = data_node.size();
     if (nelems == 0)
+    {
         m = Mat();
+        return;
+    }
     CV_Assert(nelems == m.total()*m.channels());
 
     data_node.readRaw(dt, (uchar*)m.ptr(), m.total()*m.elemSize());

modules/core/test/test_io.cpp

@@ -2043,7 +2043,10 @@ void testExactMat(const Mat& src, const char* ext)
     EXPECT_EQ(dst.empty(), srcIsEmpty);
     EXPECT_EQ(src.dims, dst.dims);
     EXPECT_EQ(src.size, dst.size);
-    EXPECT_EQ(cv::norm(src, dst, NORM_INF), 0.0);
+    if (!srcIsEmpty)
+    {
+        EXPECT_EQ(0.0, cv::norm(src, dst, NORM_INF));
+    }
 }
 
 typedef testing::TestWithParam<const char*> FileStorage_exact_type;