[BUG] Adds null to the Param which should actually be None and the attribute error. in api

It seems that openapi cli gererates invalid python test code

Steps to reproduce

commands

openapi-generator-cli generate 
-i api-doc.json \
-g python \
-o out/python
cd /out/python
tox

Results

1. No atribute errors, for some reason tests think that banana_tick_bananars should have attribute bananaTick_bananars but generated code does not have that and it would be silly as api definition does not have nested bananaTickBananars.

...
        else :                                                                                                          
            return bananaTick(                                                                                        
                id = '0',                                                                                               
                name = '0',
                bananars = [                                                                                             
>                   openapi_client.models.banana_tick_bananars.bananaTick_bananars(                                   
                        provided_bananas = [                                                                             
                            openapi_client.models.provided_banana.Providedbanana(
                                snapshot = True,
                                banana = '0', )                                                                          
                            ],
                        params = null,
                        type = null,                                                                                    
                        name = '0',                                                                                     
                        id = null, )
                    ],                                                                                                  
        )                                                                                                               
E           AttributeError: module 'openapi_client.models.banana_tick_bananars' has no attribute 'bananaTick_bananars'

2. Why genererator uses null in python code and not None?

...
        # model = openapi_client.models.tracer_res.BananarRes()  # noqa: E501
        if include_optional :                                               
            return BananarRes(                                               
                id = null,                                                  
                name = '0',                                                 
                type = null,                                                
                availability = 1.337                                        
            )                                                               
        else :                                                              
            return BananarRes(                                               
>               id = null,                                                  
                name = '0',                                                 
                type = null,                                                
                availability = 1.337,                                       
        )                                                                   
E           NameError: name 'null' is not defined                           

Reproduce the issue with openapi-generator-cli using the shown api-doc.json, the python generator, and tox. Inspect the generated Python model and test code for the invalid model attribute name and JSON-style null values; done means the generated tests use existing model attributes and valid Python None values.