Merge pull request #25660 from Abdurrahheem:ash/fix-slice-empty-input

Slice layer parser fix to support empty input case #25660

This PR fixes Slice Layer's parser to handle empty input cases (cases with initializer)
It fixed the issue rased in #24838

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [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/dnn/src/onnx/onnx_importer.cpp

@@ -1202,7 +1202,12 @@ void ONNXImporter::parseReduce(LayerParams& layerParams, const opencv_onnx::Node
 
 void ONNXImporter::parseSlice(LayerParams& layerParams, const opencv_onnx::NodeProto& node_proto)
 {
-    MatShape inpShape = outShapes[node_proto.input(0)];
+    MatShape inpShape;
+    if (constBlobs.find(node_proto.input(0)) != constBlobs.end())
+        inpShape = shape(getBlob(node_proto, 0));
+    else {
+        inpShape = outShapes[node_proto.input(0)];
+    }
     int dims = inpShape.size();
     std::vector<int> begin(dims, 0);
     std::vector<int> end(dims, INT_MAX);