Return null when handling command result

[jivanf]

When a command result is handled in HandleCommandResult, a TerminalNode is always returned, even if the command's result is null. This prevents execution from being passed through to other user-defined commands and the game's command parser. There's also a comment here indicating that HandleCommandResult should return TerminalNode or null, but currently null is never returned.

This PR simply adds a condition in HandleCommandResult to return null when the command's result is null and fixes a typo in the comment mentioned previously.