fix: allow None as a column type

[betodealmeida]

Tentative fix for #113.

When creating a virtual dataset from a dbt model in Snowflake we're getting the following error:

[[09:32:48]] ERROR: preset_cli.cli.superset.sync.dbt.datasets: Unable to create dataset                                                                                                           datasets.py:99
                    Traceback (most recent call last):                                                                                                                                                                              
                      File "/usr/local/lib/python3.9/site-packages/preset_cli/cli/superset/sync/dbt/datasets.py", line 97, in sync_datasets                                                                                         
                        dataset = create_dataset(client, database, model)                                                                                                                                                           
                      File "/usr/local/lib/python3.9/site-packages/preset_cli/cli/superset/sync/dbt/datasets.py", line 63, in create_dataset                                                                                        
                        return client.create_dataset(**kwargs)                                                                                                                                                                      
                      File "/usr/local/lib/python3.9/site-packages/preset_cli/api/clients/superset.py", line 534, in create_dataset                                                                                                 
                        elif column["type"].lower() == "string":                                                                                                                                                                    
                    AttributeError: 'NoneType' object has no attribute 'lower'                                  

Which suggests that when running SELECT * FROM {source} to infer the column names and types in the model at least one of the columns has the type set to None (coming from inspect.get_columns, so it's being set by the Snowflake driver).

I fixed it by checking for this case, and setting the column type to the most generic when this happens.