QmlJS tooltips: Avoid unhelpful tooltips.

Saying 'unknown' or 'undefined' isn't very useful.

Change-Id: I130013a8f025ebc5eb984358f269fdf8a23aae92
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/plugins/qmljseditor/qmljshoverhandler.cpp

@@ -305,10 +305,11 @@ void HoverHandler::prettyPrintTooltip(const QmlJS::Value *value,
     }
 
     if (toolTip().isEmpty()) {
-        QString typeId = context->valueOwner()->typeId(value);
-        if (typeId != QLatin1String("undefined"))
+        if (!value->asUndefinedValue() && !value->asUnknownValue()) {
+            const QString typeId = context->valueOwner()->typeId(value);
             setToolTip(typeId);
         }
+    }
 }
 
 // if node refers to a property, its name and defining object are returned - otherwise zero