`wp core config` fails on any empty parameter

Steps to reproduce:

```
rm wp-config.php
wp core config --dbname=''
```

Expected output:

```
Warning: missing --dbuser parameter
usage: wp core config --dbname=<name> --dbuser=<user> [--dbpass=<password>] [--dbhost=<host>] [--dbprefix=<prefix>]
```

Actual output:

```
Error: The site you have requested is not installed.
Run `wp core install`.
```

Previously: #333