`apply` should throw if it infers an out type of `Union{}`

[tpgillam]

If the output type inferred is the nadir type Union{}, that is a likely indication that the types of the argument have no appropriate method that can be dispatched.

In this case, we should throw an exception immediately.

It is already possible to specify the out_type manually, which is hopefully sufficient to work around for more specific user needs.