Preserve defaults for zero initialised structs as well

[yhabteab]

Before submitting your PR, please confirm the following.

• Describe the purpose for which you created this PR.
  With Preserve defaults while decoding nested structs #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"

• 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 ✅

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

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Additional details and impacted files

@@            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              

[goccy]

Thank you for your contribution ! LGTM 👍