I fixed the type issues introduced in 6.3.0. All helpers have a return type of string once again.

[!IMPORTANT] To resolve long-existing type issues, I had to change an implementation detail: In certain error cases, the helpers now return an empty string '' instead of undefined.

Since both values are considered falsy in *.hbs as well as *.{js,ts}, I'm hoping that changing the return value will be non-breaking in most cases.

Going forward, we'll try to limit the API and be more strict about types.