feat: make finish reason nullable in json marshal

[yuikns]

Describe the change

this PR added a MarshalJSON to FinishReason.

Describe your solution

MarshalJSON implemented Marshaler interface in package encoding/json

If FinishReason is FinishReasonNull or empty string, we can render it as null instead of "null".

Tests

c := &ChatCompletionChoice{
	FinishReason: FinishReasonNull,
}
jsonBytes, _ := json.Marshal(c)
fmt.Println(string(jsonBytes)) // it will print { .... "finish_reason": null }

Since i am not sure is this PR expected, i didn't add a lot of test cases in *_test.go file in the PR. but it was test locally.

[codecov]

Codecov Report

Merging #449 (00f38ad) into master (1876e0c) will increase coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #449      +/-   ##
==========================================
+ Coverage   97.03%   97.05%   +0.02%     
==========================================
  Files          17       17              
  Lines         709      714       +5     
==========================================
+ Hits          688      693       +5     
  Misses         15       15              
  Partials        6        6              

Files Changed	Coverage Δ
chat.go	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

[vvatanabe]

@yuikns Thank you for patiently discussing! I merged!