storage_service: handle empty token list correctly
storage_service: handle empty token list correctly

boost::split() return one empty string if called on an empty input.
Trying to cast an empty string to a token value results in a bad_lexical_cast
exception. Fix it by handling empty token list explicitly.

Message-Id: <c76d79847e777c7f90e45c3a1fc537c0d830755a@scylladb.com>

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