fix: skip encoding an inline field if it is null

[zoncoen]

The current implementation returns an error if an inline field is null. This PR makes skipping encoding null inline fields.

This behavior is the same as encoding/json.

b, err := json.Marshal(struct {
	*base `yaml:",inline"`
	C     bool
}{
	C: true,
})
if err != nil {
	panic(err)
}
fmt.Println(string(b))

=> {"C":true}

[codecov-commenter]

Codecov Report

Merging #386 (d90e6f3) into master (42fb764) will increase coverage by 0.05%.
The diff coverage is 100.00%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
+ Coverage   75.61%   75.66%   +0.05%     
==========================================
  Files          13       13              
  Lines        4650     4652       +2     
==========================================
+ Hits         3516     3520       +4     
+ Misses        874      873       -1     
+ Partials      260      259       -1     

[goccy]

Thank you for your great PR !! LGTM 👍