refactor: remove null check in register.ts options.base

Inside registerServiceWorker, we were originally using the nullash coalescing
operator to check if options.base was null or undefined. However, I realized
this check is not necessary.

If you look at getOptions' return value, we return an object with a key "base"
which is of type "string". We get that value by calling resolveBase which always
returns a string.

As a result, we didn't need to check if options.base was null or undefined
because it never can be.