[ONNX] Fix editor tests to check for nullptr before dereferencing

[mvafin]

Details:

• Fix segmentation faults when running editor tests with onnx delegate

Tickets:

• ticket-id

[Copilot]

Pull Request Overview

This PR addresses segmentation faults in ONNX editor tests by adding null pointer checks before calling methods on potentially null objects. The changes ensure that tests fail gracefully with informative error messages rather than crashing.

Key Changes

• Added ASSERT_TRUE checks to verify pointers are not null before dereferencing
• Added ASSERT_GE checks to ensure containers have sufficient elements before indexing
• Refactored chained method calls into separate variables with intermediate null checks

Comments suppressed due to low confidence (1)

src/frontends/onnx/tests/onnx_editor.cpp:453

• Test name appears truncated: 'subgraph__twice_input_edge_from_tensor_with_single_co' should likely be 'subgraph__twice_input_edge_from_tensor_with_single_consumer' or similar.

OPENVINO_TEST(onnx_editor, subgraph__twice_input_edge_from_tensor_with_single_consumer) {

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[bumbosiepsak]

Looks good and has quality messages.