job: abort if both service and key are empty

From the job description both, service name and key can be empty
strings. Since in that case there is no way to identify more than one
entry assume this is invalid and return an error

Author: akallabeth

qtkeychain/keychain.cpp

@@ -79,6 +79,10 @@ void Job::emitFinishedWithError(Error error, const QString &errorString)
 
 void Job::scheduledStart()
 {
+    if (d->service.isEmpty() && d->key.isEmpty()) {
+        emitFinishedWithError(EntryNotFound, tr("Both service name and key are empty"));
+        return;
+    }
     d->scheduledStart();
 }