Prevent crash when reloading renderable that only has labels and no data

Occurred when unchecking and rechecking the "Draw elements" property for example the "Eclipltic Sphere Labels"

modules/digitaluniverse/rendering/renderablebillboardscloud.cpp

@@ -1492,6 +1492,11 @@ void RenderableBillboardsCloud::createDataSlice() {
     ZoneScoped
 
     _slicedData.clear();
+
+    if (_fullData.empty() || _nValuesPerAstronomicalObject == 0) {
+        return;
+    }
+
     if (_hasColorMapFile) {
         _slicedData.reserve(8 * (_fullData.size() / _nValuesPerAstronomicalObject));
     }