/api/updateLocations should be able to set provider to null

[Simon Willison]

Currently that API, documented at https://vial.calltheshots.us/api/docs#post-apiupdatelocations, requires you to send a provider_name and provider_type - but there's no mechanism to set provider back to null.

Originally posted by @simonw in #705 (comment)

[Simon Willison]

I'm going to offer "provider_null": true as the option here - and you get an error if you also send a provider_name or provider_type.

[Simon Willison]

Documentation is here: https://github.com/CAVaccineInventory/vial/blob/ffff6a1ee7e8fc14f17cd2c8456022fbc14e68a3/docs/api.md#post-apiupdatelocations