Throw the correct error when second argument to ow is undefined

[fnesveda]

When calling ow('value', undefined, ow.string), ow threw an unhelpful error:

TypeError: Cannot read property 'Symbol(test)' of undefined
      at isPredicate
...

because it tried to access a property on an undefined value.

This fixes it so that it throws the right error (as with other, non-undefined values):

Expected second argument to be a predicate or a string, got `undefined`