BUG: Fix GetStorableNodesModifiedSinceRead for short text nodes

If a vtkMRMLTextNode contains short text then it does not use a storage node to store that to avoid creating many small text nodes.
However, the vtkMRMLScene::GetStorableNodesModifiedSinceRead method detected a storable node that does not have a storage node as a node that has storable content modified.
Therefore it always returned "true" if a text node with short text was present in the scene, triggering unnecessary warning popups on scene close or exit.

Fixed the logic in vtkMRMLScene::GetStorableNodesModifiedSinceRead to correctly handle storable nodes that do not need storage nodes.

Libs/MRML/Core/vtkMRMLScene.cxx

@@ -3744,6 +3744,12 @@ ::GetStorableNodesModifiedSinceRead(vtkCollection* modifiedStorableNodes)
     if (!storableNode->GetHideFromEditors() &&
          storableNode->GetModifiedSinceRead())
     {
+      if (!storableNode->GetStorageNode() && storableNode->GetDefaultStorageNodeClassName().empty())
+      {
+        // The storable node does not have a storage node, but it does not need one (because content is stored in the scene
+        // (for example vtkMRMLTextNode containing short text).
+        continue;
+      }
       found = true;
       if (modifiedStorableNodes)
       {