Passing empty string arguments Issue #3

The possibility to have mysql credentials as a parameter is a great idea but in our case, we want to use credentials stored in my.cnf.
Because ${1:-default} treat an empty string argument as non-valid, this cases are impossible at this time:

```
# Lease empty in order to mysql uses my.cnf to credentials
clustercheck "" ""
```

```
# User clustercheck with empty password
clustercheck "clustercheck" ""
```

I don't know what solution would be the most elegant... Any idea?

(sorry for the duplicate issue, I'm a Github newbie)