JsonWizard: Allow for wizards without generators

The Checkout wizard will not feature any generator.

Change-Id: I05900309a5ea8a638493ab0fa5499759bcdc181f
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/projectexplorer/jsonwizard/jsonwizardfactory.cpp

@@ -557,7 +557,7 @@ bool JsonWizardFactory::initialize(const QVariantMap &data, const QDir &baseDir,
 
     // Generator:
     QVariantList list = objectOrList(data.value(QLatin1String(GENERATOR_KEY)), errorMessage);
-    if (list.isEmpty()) {
+    if (!errorMessage->isEmpty()) {
         *errorMessage = tr("When parsing \"generators\": %1").arg(*errorMessage);
         return false;
     }