Fixed Bug in Which Mapper Can't Map to ListConfig if Input is None

[NdubisiOnuora]

Example:
if value of config is the following

ListConfig('example.food', {'item': 'name'})

And input is

{'example': {'food': None}}

The following error is shown
TypeError: 'NoneType' object is not iterable

This fix just returns an empty list if the input is None.

[coveralls]

Coverage remained the same at 100.0% when pulling 8a8ed0a on NdubisiOnuora:fix_list_config_null into 348b6cb on dealertrack:master.

[NdubisiOnuora]

@miki725, please review and merge.

[miki725]

can you update HISTORY.rst as well?

[NdubisiOnuora]

Done

[NdubisiOnuora]

@miki725: whenever you're ready to re-review and merge.

[miki725]

wrap TyperError around double backticks to render as code. same for None

[NdubisiOnuora]

Done

[NdubisiOnuora]

@miki725, all changes have been made.

[NdubisiOnuora]

@miki725: ping.

[NdubisiOnuora]

Yaissssssssssss!