gui: use fallback value for Version and User Agent during peer connection

During connection setup for a peer, getpeerinfo returns "version": 0, "subver": ""
and the GUI Peers window displays 0 and an empty field, respectively.

Give these fields the same behavior as the other fields in the GUI Peers window:
display the fallback value in src/qt/forms/debugwindow.ui (i.e. "N/A") until a
valid result is available after the peer connection completes.

Author: jonatack

src/qt/rpcconsole.cpp

@@ -1178,8 +1178,12 @@ void RPCConsole::updateDetailWidget()
     ui->peerPingTime->setText(GUIUtil::formatPingTime(stats->nodeStats.m_last_ping_time));
     ui->peerMinPing->setText(GUIUtil::formatPingTime(stats->nodeStats.m_min_ping_time));
     ui->timeoffset->setText(GUIUtil::formatTimeOffset(stats->nodeStats.nTimeOffset));
-    ui->peerVersion->setText(QString::number(stats->nodeStats.nVersion));
-    ui->peerSubversion->setText(QString::fromStdString(stats->nodeStats.cleanSubVer));
+    if (stats->nodeStats.nVersion) {
+        ui->peerVersion->setText(QString::number(stats->nodeStats.nVersion));
+    }
+    if (!stats->nodeStats.cleanSubVer.empty()) {
+        ui->peerSubversion->setText(QString::fromStdString(stats->nodeStats.cleanSubVer));
+    }
     ui->peerConnectionType->setText(GUIUtil::ConnectionTypeToQString(stats->nodeStats.m_conn_type, /*prepend_direction=*/true));
     ui->peerNetwork->setText(GUIUtil::NetworkToQString(stats->nodeStats.m_network));
     if (stats->nodeStats.m_permission_flags == NetPermissionFlags::None) {