Preserve defaults for zero initialised structs as well

[yhabteab]

Before submitting your PR, please confirm the following.

• [x] Describe the purpose for which you created this PR. With https://github.com/goccy/go-yaml/pull/260, go-yaml already supports preserving default values for nested structs when decoding, however, there's one use case that was missed.

---
first: "Test"
nested: # The decoder will always zero-initialise this nested struct and override the defaults
  # Just some comment here
#  val: "default"

• [x] Create test code that corresponds to the modification

fixes #441

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.78%. Comparing base (237df0e) to head (092b488).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #442 +/- ## ========================================== + Coverage 78.77% 78.78% +0.01% ========================================== Files 13 13 Lines 4004 4007 +3 ========================================== + Hits 3154 3157 +3 Misses 591 591 Partials 259 259 ```