Feature/improve gs import

[frederic-reis]

Some improvements in the document_import_from_google_spreadsheet command:

• Replace empty string by null for empty value in the JSON. Empty strings caused validation errors for integer, date, etc....
• Add --force argument to don't exit when there is a validation error
• Add --ignore-empty-value argument to don't include empty values in the JSON. To use with --reset_data argument on the document_import_from_json command.

Allow to create previews for all documents in document_create_snapshot command.

[frederic-reis]

@danieleguido Tu peux jeter un oeil et merger si ça te convient ?