title: "type object argument after * must be an iterable, not NoneType" error using RxSqlServerData & rx_featurize while inserting data into the database
body: <p>I am using SQL Server 2017 in-database Machine Learning Services with python (<code>revoscalepy</code>,<code>microsoftml</code>) for creating a model with Jupyter Server notebook. 
  I am able to set my compute_context using revoscalepy and successfully run a model and store the results into a dataframe.
  I am now trying to store insert(or write) these dataframe values using the same connection-string that I used while connecting to the database using <code>rx_featurize</code> but I get this error <code>type object argument after * must be an iterable, not NoneType</code>.</p>
  
  <p>Below is the code I am running :</p>
  
  <pre><code>output_df = pd.DataFrame(data = predictions, index=unique_id, columns=['predictions'])
  
  from microsoftml import rx_featurize
  
  rx_featurize(data=output_df,output_data=RxSqlServerData(connection_string=connection_string_1, table = 'predicted', database_name='banktest'), overwrite = True)
  </code></pre>
  
  <p>Error is as below :</p>
  
  <pre><code>TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-32-7ae26d056309&gt; in &lt;module&gt;()
        4 # a_df = pd.DataFrame([[0, 1], [2, 3]], columns=[...])
        5 
  ----&gt; 6 rx_featurize(data=output_df,output_data=RxSqlServerData(connection_string=connection_string_1, table = 'predicted', database_name='banktest'), overwrite = True)
  
  C:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\PYTHON_SERVICES\lib\site-packages\microsoftml\modules\featurize.py in rx_featurize(data, output_data, overwrite, data_threads, random_seed, max_slots, ml_transforms, ml_transform_vars, row_selection, transforms, transform_objects, transform_function, transform_variables, transform_packages, transform_environment, blocks_per_read, report_progress, verbose, compute_context)
      162     transform_nodes = transform_data(
      163         ml_transforms, data=input_data,
  --&gt; 164         features=None, output_data=output_data_, model=transform_model)
      165 
      166     ## combine the transform models
  
  C:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\PYTHON_SERVICES\lib\site-packages\microsoftml\modules\graph_composition.py in transform_data(ml_transforms, data, features, output_data, model)
       41 
       42     if features is None:
  ---&gt; 43         sub_graph = Graph(*ml_transforms)
       44     else:
       45         ## combine the features
  
  TypeError: type object argument after * must be an iterable, not NoneType
  </code></pre>
  
  <p>microsoftml is a library which is installed when I installed python services.</p>
  
  <p>Here's the link as to why I took <code>rx_featurize</code> to insert data into the database.
  [<a href="https://stackoverflow.com/questions/47696221/using-revoscalepy-to-insert-data-into-a-database]">Using revoscalepy to insert data into a database</a></p>
  
  <p>Also I created a blank table predicted with the columns as per dataframe but still error showed</p>