Fixed Need to make checks that the callback is a proper function and eventName is a proper value as well #2

Fixed Null/undefined for param values to 'on' and 'once' methods.
What happens if I pass eventName as true, or 5, or 1.78 or -800 or { a: null } ?
Maybe it makes sense to have a check for eventName to be a string? Not sure how the examples ^ will work