Fix a problem with empty yaml files

This is a fix for something that I started seeing when an empty configuration file was added in `cli_spec.rb`, but only on ruby 1.9.3p194. 

```
Failures:

  1) Rubocop::CLI configuration from file when a file inherits from the old auto generated file prints no warning when --auto-gen-config is not set
     Failure/Error: expect { cli.run(%w(-c .rubocop.yml)) }.not_to exit_with_code(1)
     Psych::SyntaxError:
       (/tmp/d20140511-12757-h8w3tr/work/rubocop-todo.yml): control characters are not allowed at line 1 column 1
     # ./lib/rubocop/config_loader.rb:27:in `load_file'
     # ./lib/rubocop/config_loader.rb:67:in `block in base_configs'
     # ./lib/rubocop/config_loader.rb:58:in `map'
     # ./lib/rubocop/config_loader.rb:58:in `base_configs'
     # ./lib/rubocop/config_loader.rb:108:in `resolve_inheritance'
     # ./lib/rubocop/config_loader.rb:30:in `load_file'
     # ./lib/rubocop/config_store.rb:23:in `options_config='
     # ./lib/rubocop/cli.rb:60:in `act_on_options'
     # ./lib/rubocop/cli.rb:27:in `run'
     # ./spec/rubocop/cli_spec.rb:1963:in `block (5 levels) in <top (required)>'
     # ./spec/rubocop/cli_spec.rb:1963:in `block (4 levels) in <top (required)>'
     # ./spec/support/isolated_environment.rb:27:in `block (4 levels) in <top (required)>'
     # ./spec/support/isolated_environment.rb:26:in `chdir'
     # ./spec/support/isolated_environment.rb:26:in `block (3 levels) in <top (required)>'
     # ./spec/support/isolated_environment.rb:8:in `block (2 levels) in <top (required)>'
```

It doesn't happen in the Travis builds, so it varies between installations.