Empty seperator does not work as option

` const input = "-ä ß üö,.";`
` const separator = "";`
` let res = getSlug(input, { separator: separator});`
` console.log(res); // ae-ss-ueoe`
` res = getSlug(input, separator);`
` console.log(res); // -aessueoe`

When the seperator is not empty it gives correct results (-ae+ss+ueoe).