wallet: fix legacy spkm default birth time

To avoid scanning blocks, as assumed by a wallet with no
generated keys or imported scripts, the default value for
the birth time needs to be set to the maximum int64_t value.

Once the first key is generated or the first script is imported,
the legacy SPKM will update the birth time automatically.

src/wallet/scriptpubkeyman.cpp

@@ -709,7 +709,7 @@ void LegacyScriptPubKeyMan::UpdateTimeFirstKey(int64_t nCreateTime)
         // Cannot determine birthday information, so set the wallet birthday to
         // the beginning of time.
         nTimeFirstKey = 1;
-    } else if (!nTimeFirstKey || nCreateTime < nTimeFirstKey) {
+    } else if (nTimeFirstKey == UNKNOWN_TIME || nCreateTime < nTimeFirstKey) {
         nTimeFirstKey = nCreateTime;
     }