gist.py: Set the description property in construct_data's data as ''
gist.py: Set the description property in construct_data's data as ''

When `--description` property is not used, it is set as Nil in the data,
hence, the API call fails.

Closes https://github.com/meetmangukiya/gist-cli/issues/3

meetmangukiya/gistey,meetmangukiya/gistey