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 #13110.

Github-Pull: #13265
Rebased-From: b0d2ca9fb66d793e3c0f2e6ede811f1b16c33a9f
Tree-SHA512: 67e446e9ced901e37003a9661b6abea268e2ea648ac3b076d91c8d996de96bed389839a09d579a6562d930bcf501a091eb67454f474aae1174108a8650502072

src/wallet/wallet.cpp

@@ -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)