Add validation to check that parameters aren't nullish (#205)

Validation has been added to the signing and encryption functions to
ensure that the parameters are not nullish. This should help with
migrating from the last major version, as it would be easy to
accidentally use the wrong parameter name when migrating.

A nullish check was used rather than a falsy check because there are
falsy values that are valid for some parameters (e.g. the data being
signed can be `0`). `null` and `undefined` are never valid in these
cases though.