Good question. If we allow it and change the return shape accordingly, this will be a very disruptive breaking change. IMO the benefit is very marginal to justify the change: both empty string and comment only input is not the typical expected input for parseExpression, but all users now have to access expression property for the parsed AST.

The JSONC parsing is an interesting use case, I think the current prettier approach is good enough since such input is not the popular JSONC input either. Would it help prettier if we throw a new error for comment-only / empty string in parseExpression? So that you don't have to depend on the general unexpected token error.