QmlJS: Fix possible crash

For a valid AST id should be never empty.
A crash has been reported. though. Most likely the parser
injected a UiQualifiedId for an incomplete document.

Change-Id: Id2f1b25cf9d78982524d7aaff2b43210e1ae2a3b
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>

src/libs/qmljs/qmljscheck.cpp

@@ -719,6 +719,10 @@ void Check::endVisit(UiObjectInitializer *)
 void Check::checkProperty(UiQualifiedId *qualifiedId)
 {
     const QString id = toString(qualifiedId);
+
+    if (id.isEmpty())
+        return;
+
     if (id.at(0).isLower()) {
         if (m_propertyStack.top().contains(id))
             addMessage(ErrPropertiesCanOnlyHaveOneBinding, fullLocationForQualifiedId(qualifiedId));