Get rid of annoying error message

The following error message appears multiple times when running
`./dev setup-unit-tests --update-gem-cache`:

````
  crowbar/./parse_yml.rb:7: undefined method `[]' for nil:NilClass (NoMethodError)
    from crowbar/./parse_yml.rb:6:in `each'
    from crowbar/./parse_yml.rb:6
````

This patch prevents that.