[ONNX] check if 3rd input to Pad op is not null (#12797)

src/core/tests/models/onnx/pad_optional_constant.prototxt

@@ -0,0 +1,61 @@
+ir_version: 6
+producer_name: "backend-test"
+graph {
+  node {
+    input: "x"
+    input: "pads"
+    input: ""
+    output: "y"
+    op_type: "Pad"
+    attribute {
+      name: "mode"
+      s: "constant"
+      type: STRING
+    }
+  }
+  name: "test_constant_pad"
+    initializer {
+    dims: 4
+    data_type: 7
+    int64_data: 0
+    int64_data: 2
+    int64_data: 0
+    int64_data: 0
+    name: "pads"
+  }
+  input {
+    name: "x"
+    type {
+      tensor_type {
+        elem_type: 1
+        shape {
+          dim {
+            dim_value: 3
+          }
+          dim {
+            dim_value: 2
+          }
+        }
+      }
+    }
+  }
+  output {
+    name: "y"
+    type {
+      tensor_type {
+        elem_type: 1
+        shape {
+          dim {
+            dim_value: 3
+          }
+          dim {
+            dim_value: 4
+          }
+        }
+      }
+    }
+  }
+}
+opset_import {
+  version: 11
+}

src/frontends/onnx/frontend/src/op/pad.cpp

@@ -14,6 +14,7 @@
 #include "ngraph/op/convert.hpp"
 #include "ngraph/op/util/op_types.hpp"
 #include "ngraph/shape.hpp"
+#include "onnx_import/core/null_node.hpp"
 #include "op/pad.hpp"
 #include "utils/convpool.hpp"
 
@@ -64,14 +65,15 @@ OutputVector pad(const Node& node) {
 }  // namespace set_1
 namespace set_11 {
 OutputVector pad(const Node& node) {
-    auto data = node.get_ng_inputs().at(0);
-    auto pads = node.get_ng_inputs().at(1);
+    const auto inputs = node.get_ng_inputs();
+    const auto& data = inputs[0];
+    const auto& pads = inputs[1];
     Output<ngraph::Node> values;
     Output<ngraph::Node> padding_begin;
     Output<ngraph::Node> padding_end;
 
-    if (node.get_ng_inputs().size() == 3) {
-        values = node.get_ng_inputs().at(2);
+    if (inputs.size() == 3 && !ngraph::op::is_null(inputs[2])) {
+        values = inputs[2];
     } else {
         values = default_opset::Constant::create(data.get_element_type(), ngraph::Shape{}, {0});
     }

src/frontends/tests/onnx/onnx_import.in.cpp

@@ -3316,6 +3316,19 @@ NGRAPH_TEST(${BACKEND_NAME}, onnx_model_pad_constant) {
     test_case.run();
 }
 
+NGRAPH_TEST(${BACKEND_NAME}, onnx_model_pad_optional_constant) {
+    const auto function = onnx_import::import_onnx_model(file_util::path_join(CommonTestUtils::getExecutableDirectory(),
+                                                                              SERIALIZED_ZOO,
+                                                                              "onnx/pad_optional_constant.onnx"));
+    auto test_case = test::TestCase(function, s_device);
+
+    test_case.add_input<float>({1.f, 1.2f, 2.3f, 3.4f, 4.5f, 5.7f});
+    test_case.add_expected_output<float>(Shape{3, 4},
+                                         {0.f, 0.f, 1.f, 1.2f, 0.f, 0.f, 2.3f, 3.4f, 0.f, 0.f, 4.5f, 5.7f});
+
+    test_case.run();
+}
+
 NGRAPH_TEST(${BACKEND_NAME}, onnx_model_pow_float32_float32) {
     const auto function = onnx_import::import_onnx_model(file_util::path_join(CommonTestUtils::getExecutableDirectory(),
                                                                               SERIALIZED_ZOO,