When we generate a `:common_name` on Users, we check for the presence of
both `first_name` and `last_name`. If either is `nil`, we don't generate
a `common_name`.

ApiKey users only have one name (the name of the ApiKey itself) but we
should set the `last_name` to an empty string. This way, the common name
will still be generated.

Because the `metabase.models.user/insert-new-user!` function requires a
NonBlankString for `first_name` and `last_name` if they're present, I
went back to calling `t2/insert-returning-instances!` directly.