primary key can be changed if index is empty

[dichotommy]

As discussed in this slack message (private link) and this PR comment, it is now possible to change the primary key of an empty index using the update index endpoint.

Before, you could only do this if the index had never possessed a primary key.

I'm not sure which pages are affected by this change, but certainly the update index endpoint (linked above) could be clarified, and possibly also the primary key guide.

[curquiza]

FYI there is the fresh new spec defining the behavior here: https://github.com/meilisearch/specifications/blob/develop/text/0132-indexes-api.md#324-put---indexesindex_uid

@gmourier FYI