fix crash during refresh feature discovery

src/main/java/eu/siacs/conversations/services/XmppConnectionService.java

@@ -4880,6 +4880,7 @@ public class XmppConnectionService extends Service {
         if (result != null) {
             return result;
         } else {
+            if (key.first == null || key.second == null) return null;
             result = databaseBackend.findDiscoveryResult(key.first, key.second);
             if (result != null) {
                 discoCache.put(key, result);