message: MutableStorage::commit() returns error if no blocks were applied
diff: @@ -141,7 +141,10 @@ namespace iroha {
  assert(not committed);
  return "Tried to commit mutable storage twice.";
  }
  + if (not ledger_state_) {
  assert(ledger_state_);
  + return "Tried to commit mutable storage with no blocks applied.";
  + }
  try {
  sql_ << "COMMIT";
  committed = true;
  