text: Title: Fillna does not work if fields_group is not None
  Body: ## 🐛 Bug Description
  
  <!-- A clear and concise description of what the bug is. -->
  The Fillna processor does not work if fields_group is not None since assigning values to df.values changes nothing.
  
  ## To Reproduce
  
  Use any model and specify fields_group for Fillna processor.
  
  ## Expected Behavior
  
  <!-- A clear and concise description of what you expected to happen. -->
  No nan after calling Fillna.
  
  ## Additional Notes
  
  <!-- Add any other information about the problem here. -->
  Same as the issue here: https://github.com/microsoft/qlib/issues/1307#issuecomment-1785284039.
  