wp core multisite-install doesn't add --admin_user to site_admin

I found out that when I created new site with `wp core multisite-install` it somehow sets empty string into `get_option('site_admins);'.

Because It saves empty string into that option also:

```
wp super-admin add admin
```

is broken.

`super-admin` subcommand should check if that's not array and add array over there.

_Versions:_

```
$ wp core version
4.6
$ wp cli version
WP-CLI 0.24.1
```
