Ignore empty string for optional args

[habara-k]

This resolves #464 with PR line/line-openapi#70

Changes

We are customizing SDK code generation to generate code that directly passes arguments to functions, instead of using a request body struct for the API.
As a result, we are unable to use the "omitempty" tag to ignore fields with empty strings.

To address this issue, we will modify the code to exclude optional string fields from url.Values when they are empty.

[mokuzon]

@habara-k Could you also add the auto-generated code to the PR in a separate commit?
(I understand it is auto-generated during the release process, but I would like to see the differences now)