fix new CI rule switch fallthrough (#7568)

src/libxrpl/tx/transactors/Sponsor/SponsorshipTransfer.cpp

@@ -221,6 +221,7 @@ getLedgerEntrySponsorField(T const& sle, AccountID const& owner)
             }
             // LCOV_EXCL_START
             UNREACHABLE("Should not happen. Owner should be checked before calling this function.");
+            return sfSponsor;
             // LCOV_EXCL_STOP
         }
         default: