message: wallet: Exit SyncMetaData if there are no transactions to sync
  Instead of crash with an assertion error, simply exit the function
  `SyncMetaData` if there is no metadata to sync.
  Fixes
  Github-Pull:
  Rebased-From:
  Tree-SHA512:
diff: @@ -539,7 +539,9 @@ void CWallet::SyncMetaData(std::pair<TxSpends::iterator, TxSpends::iterator> ran
  }
  }
  - assert(copyFrom);
  + if (!copyFrom) {
  + return;
  + }
  // Now copy data from copyFrom to rest:
  for (TxSpends::iterator it = range.first; it != range.second; ++it)
  