Makes the carousel classPrefix optional by setting the default prefix only when it is undefined (not a blank string)

This is a change contributed by @brendan-skyrkt in PR #4. Thanks!

This change will allow for blank (empty string) class prefixes, for example "scooch" and "item" instead of "prefix-scooch" or "prefix-item". If the user does not specify a class prefix, blank or otherwise, it will still default as previously to "m-".