Feature: clear contact language

[fagiothree]

At the moment in the set_contact_language operation, there is a check for the language to be non-empty:
language must be non-empty for set_contact_language

However, from the RapidPro interface it's possible to clear the language variable. The action in the rapidpro json doesn't have have language key, i.e.
{
"uuid": "6523f537-a046-4d36-b595-a0067fa2e5c9",
"type": "set_contact_language"
}

instead of

{
"uuid": "4fc2803f-04ad-4bc2-80a8-d1c3d896412f",
"type": "set_contact_language",
"language": "eng"
}

Should we also allow for it, but maybe give a warning?