Yes there is a principled reason. If we accept that we have a list which is recursively defined using a binary cell aggregate structure, as a right-leaning tree shape, then it is advantageous to have an atomic, unique empty list at the bottom of the recursion. That atomic empty list can be represented as a machine word. We can perform a single comparison to detect the terminator: is it that object or not? Anything else, though workable, is a gratuitous complication.

The empty string is mutable. Even the literal one is potentially mutable: you can try it at your own risk. You ca make a mutable empty string with (make-string 0) or by mutating a non-empty string.

Any mutable string can be mutated to make it empty:

   (delete #\a (make-string 3 :initial-element #\a)) -> ""

Since strings aren't linked structures with a terminator, the comparison is moot.