Handling missing choice labels

[jnm]

[edit] This is not a regression, but kpi inconsistently sends us both None and the empty string to represent a label that lacks a translation, e.g. either 'label': ['choose jif', None], or 'label': ['choose jif', ''], might be sent.

Question: should formpack treat these identically? It would then fall back to the choice name if the label is an empty string. That's what happens already if the label is None.

Abbreviated schema for context:

{
    'content': {
        'schema': '1',
            {
                'name': 'choosey_moms',
                'type': 'select_one',
                '$kuid': 'nBTaqfqN3',
                'label': ['choosey moms', 'las madres exigentes'],
                '$qpath': 'choosey_moms',
                '$xpath': 'choosey_moms',
                'required': False,
                'select_from_list_name': 'vl8ap24',
            },
        ],
        'choices': [
            {
                'name': 'choose_jif',
                '$kuid': 'T06KYSoia',
                'label': ['choose jif', None],
                'list_name': 'vl8ap24',
            },
            {
                'name': 'choose_gif',
                '$kuid': '91CVcUFYb',
                'label': ['choose GIF', 'elige GIF'],
                'list_name': 'vl8ap24',
            },
        ],
        'translated': ['label'],
        'translations': ['English (en)', 'Español (es)'],
    },
}

Internal discussion: https://chat.kobotoolbox.org/#narrow/stream/4-Kobo-Dev/topic/formpack.20and.20missing.20labels/near/173165