fix(scripts/cassandra-harry): Fixing bash if condition

fix(scripts/cassandra-harry): Fixing bash if condition

The correct way to check if variable is set is:
```bash
if [ -z ${var+x} ]; then
   echo "var is unset"
else
   echo "var is set to '$var'"
fi
```

For more information please check the following link:
https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash