fix: invalid schema for function 'func_name': None is not of type 'object' (#429)(#432)

[vvatanabe]

Describe the change
The problem recurs when Parameters is a value instead of a pointer. MarshalJSON in Definition does not execute MarshalJSON if Definition is a value because the receiver is a pointer.

func (d *Definition) MarshalJSON() ([]byte, error)

Refs:

• fix error 400 during function call #433 (comment)
• fix error 400 during function call #433 (comment)

error message:

 invalid schema for function 'func_name': None is not of type 'object' 

request to reproduce:

	req := openai.ChatCompletionRequest{
		Model: openai.GPT3Dot5Turbo,
		Messages: []openai.ChatCompletionMessage{
			{
				Role:    openai.ChatMessageRoleUser,
				Content: "What is the weather like in Boston?",
			},
		},
		Functions: []openai.FunctionDefinition{{
			Name: "get_current_weather",
			Parameters: jsonschema.Definition{ // <= The problem recurs when Parameters is a value instead of a pointer.
				Type: jsonschema.Object,
				Properties: map[string]jsonschema.Definition{
					"location": {
						Type:        jsonschema.String,
						Description: "The city and state, e.g. San Francisco, CA",
					},
					"unit": {
						Type: jsonschema.String,
						Enum: []string{"celsius", "fahrenheit"},
					},
				},
				Required: []string{"location"},
			},
		}},
	}

Describe your solution

• MarshalJSON method is executed whether jsonschema.Definition in Parameters is a pointer or a value
• Initialization with an empty map if Properties is nil.

Tests

• Test MarshalJSON with both value and pointer.
• add integration test for function call.

Additional context
Issue: #429, #432

[codecov]

Codecov Report

Merging #434 (df10cf3) into master (f028c28) will not change coverage.
The diff coverage is n/a.

❗ Current head df10cf3 differs from pull request most recent head f793e8b. Consider uploading reports for the commit f793e8b to get more accurate results

@@           Coverage Diff           @@
##           master     #434   +/-   ##
=======================================
  Coverage   97.02%   97.02%           
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  Files          17       17           
  Lines         705      705           
=======================================
  Hits          684      684           
  Misses         15       15           
  Partials        6        6           

[vvatanabe]

@sashabaranov Please check it.

[sashabaranov]

Thank you for the PR!