Trully allow theme OR extension file

[nlemoine]

This fixes a behaviour that's a bit annoying.

I try to set some options in the theme.yml file. But I don't want to set any options in app/config/extensions/boltuioptions.snijder.yml.

• If I delete the file, it's automatically created by the extension getConfig process (actually copied from config/config.dist.yml).
• If I leave the file empty, I get a warning Warning: Invalid argument supplied for foreach() in Config/Config.php line 54 because it passes a non array value.

This PR is a quick and dirty workaround addressing this bug.

I'm also trying to add some fields programmatically to provide a field that can be able to select dynamic content (a page, or any contenttype). Any plans to add such field type?

It's currently not very easy to achieve this (lacking public methods like addThemeTabs, addExtensionsTabs being protected).

But that's another topic. I'll come back to it later 😉

[DennisSnijder]

Woah! Never saw this PR! Good catch!