Regenerate client with nilable types

[NipunaRanasinghe]

Purpose

This PR uses the --nullable option when generating the client using the Ballerina OpenAPI tool.

• The Asana OpenAPI specification has not properly included the "nullable" property for some request and response schemas.
• Therefore, the --nullable option is used as a precaution to avoid potential data-binding issues in the runtime, which will generate all the request/response type fields with the support to handle null values.
• This workaround can be removed once Handle un-defined nillable properties  ballerina-library#4870 is addressed.

Related to ballerina-platform/ballerina-library#6238

Examples

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec
• Checked native-image compatibility

[ayeshLK]

lgtm