20.0.2 - Fix crash when boolean flag isn't specified

What's Changed

Fix crash when boolean flag isn't specified by @fpseverino in #494

If a boolean flag like --leaf or --no-leaf isn’t specified, the CLI crashes.

This reverts the behavior for boolean flags as it was before #493.

This doesn’t change the behavior for nested flag, which were the main objectives of the previous PR.

This patch was released by @fpseverino

Full Changelog: 20.0.1...20.0.2