fix: preserve default className when className is null or undefined

[Shiva903-hue]

What does this PR do?

Preserves the default react-tabs className in UncontrolledTabs when
props.className is undefined or null.

Why?

Currently, spreading { ...defaultProps, ...props } causes an explicit
undefined or null className to override the default, which breaks
backward compatibility and styling.

Maintainers confirmed this behavior in #581.

What’s included

• Guard against undefined / null overrides
• Regression tests covering both cases

Related issue

Fixes #581