I don't have a problem with nil being a symbol, but speaking of writing shorter programs, I can't think of a single time I've actually needed to coerce nil/null to a string in any language except when printing it out for debugging purposes, and even then, in Arc:
(is "nil" (tostring pr.nil)) ; because 'nil is sent to MzScheme's 'display
(is "" string.nil) ; because 'coerce special-cases 'nil to do this
...I'm taken by surprise, and I'm not sure where this discrepancy would pay off in brevity.
I suppose string-returning procedures that need to be convenient in boolean contexts can return nil instead of "" and expect anyone who calls them to wrap them up as string:foo in string contexts... but even so, I can't think of any practical examples of when doing that would be shorter than having the procedures return only strings and wrapping them up as ~empty:foo in boolean contexts.
Is there some brilliant use of (is "" string.nil) I'm not thinking of?