unnullifyTranslations() assumes translated cannot change

[jnm]

Consider that we have a form with one named language, French (fr), and only labels—no hints:

{
    "schema": "1",
    "survey": [
        {
            "type": "integer",
            "$kuid": "kv4jl55",
            "label": [
                "Au contraire!"
            ],
            "required": false,
            "$autoname": "Au_contraire"
        }
    ],
    "settings": {
        "default_language": "French (fr)"
    },
    "translated": [
        "label"
    ],
    "translations": [
        "French (fr)"
    ]
}

If we open this in the form builder, add a hint ("My brother"), and save it:

1. unnullifyTranslations() will see that translated in the asset content contains only label;
2. It will change label to label::French (fr), but it will not alter hint;
3. The front end will PATCH the asset with this content:

   {
       "settings": [
           {
               "default_language": "French (fr)"
           }
       ],
       "survey": [
           {
               "$autoname": "Au_contraire",
               "$kuid": "kv4jl55",
               "hint": "My brother",
               "label::French (fr)": "Au contraire!",
               "required": "false",
               "type": "integer"
           }
       ]
   }

4. The back end (formpack) will now treat the form as having two translations: null (from the bare hint) and French (fr):

   {
       "schema": "1",
       "survey": [
           {
               "hint": [
                   null,
                   "My brother"
               ],
               "type": "integer",
               "$kuid": "kv4jl55",
               "label": [
                   "Au contraire!",
                   null
               ],
               "required": false,
               "$autoname": "Au_contraire"
           }
       ],
       "settings": {
           "default_language": "French (fr)"
       },
       "translated": [
           "hint",
           "label"
       ],
       "translations": [
           "French (fr)",
           null
       ]
   }

What should happen? unnullifyTranslations() must recognize that changes made by the form builder can cause translated to become invalid. We should probably use a constant list of translatable columns when unnullifying a translation.

[tinok]

@jnm @magicznyleszek I just ran into this same issue on my end (added a hint to a form that had no previous hints) and noticed that the formbuilder can't be used after refreshing or reopening the form. Is this new or what you had experienced?

Should this be a separate issue?

I made a quick video in case it's helpful.
https://user-images.githubusercontent.com/673511/120410565-370f0380-c321-11eb-9865-17667eed820c.mov