fix: handle nil values in field path and update tests

[TwoStone]

Summary

Fixes a nil pointer dereference panic that occurs when GetFrom attempts to access fields from embedded structs that are nil pointers. This adds defensive nil checking when traversing field paths to safely return zero values instead of panicking.

Problem

The FieldCacheEntry.GetFrom method in value/reflectcache.go traverses field paths using FieldByIndex without checking if intermediate struct values are nil. When dealing with embedded pointer structs (e.g., *testBasicStruct embedded with ,inline tag), if the pointer is nil, calling FieldByIndex causes a panic.

Example scenario:

type testEmbeddedStruct struct {
    *testBasicStruct `json:",inline"`
}

// When testBasicStruct is nil, accessing its fields would panic
val := testEmbeddedStruct{} // embedded pointer is nil

Impact

• No breaking changes: Existing behavior preserved for non-nil paths
• Bug fix: Prevents panic when accessing fields through nil embedded struct pointers
• Graceful degradation: Returns appropriate zero values instead of crashing

[k8s-ci-robot]

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This pull-request has been approved by: TwoStone
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