Make "Content" field in "ChatCompletionMessage" omitempty

[saileshd1402]

Describe the change
According to the OpenAI API spec, the content field of ChatCompletionMessage should be nullable. Right now when we try to marshal the struct with empty string, we get "content": "", but ideally, the content key should exist if that field is empty

Provide OpenAI documentation link
https://platform.openai.com/docs/api-reference/chat/object
Under: choices -> message -> content

Describe your solution
We can fix this my adding an omitempty json tag to the "content" field in the "ChatCompletionMessage" struct

Tests
Tested this by sending a completion request with empty content field to a Nvidia NIM Inference Server

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (774fc9d) to head (7f7d2c5).
Report is 81 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #926      +/-   ##
==========================================
+ Coverage   98.46%   98.69%   +0.23%     
==========================================
  Files          24       26       +2     
  Lines        1364     1761     +397     
==========================================
+ Hits         1343     1738     +395     
- Misses         15       16       +1     
- Partials        6        7       +1     

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[Datta0]

LGTM

[sashabaranov]

Thank you!