Function Call Error Due to Nil Properties

[vvatanabe]

Describe the bug
An error occurs when using the Function Call feature. The relevant error message is as follows:

Completion error: error, status code: 400, message: Invalid schema for function 'func_name': None is not of type 'object'

This issue arises when the map in the Properties field of jsonschema.Definition is set to nil.

To Reproduce
This bug can be reproduced through the following steps:

1. Use jsonschema.Definition when constructing parameters for the Function Call.
2. Set the map in the Properties field of jsonschema.Definition to nil.
3. Send a request using the Function Call feature.

Please note that the process works correctly if the map in the Properties field of jsonschema.Definition is initialized.

PR for reproduction:
#430

Expected behavior
Even if the map in the Properties field of jsonschema.Definition is nil, it should be possible to send a request using the Function Call feature.

Environment
go-openai version: v1.13.0

Additional context
This bug was caused by changes made in pull request #419. Specifically, omitempty was removed from the Properties field of jsonschema.Definition by pull request #419. This change caused issues when the map in the Properties field was set to nil.

For detailed reasons why omitempty was removed, please refer to the description in pull request #419.

As a bug handling measure, it seems better to initialize the map in the Properties field of jsonschema.Definition to an empty map if it is set to nil. Also, since jsonschema.Definition has a recursive structure that can have jsonschema.Definition as a child, the check for nil and initialization should also be done recursively.

[vvatanabe]

Fixed #431

[vvatanabe]

The problem recurs when Parameters is a value instead of a pointer.
Refs: #432

[vvatanabe]

Fixed #434