Resolve #3375925 "Configuration page erros when loading or saving"

Closes #3375925

Just uninstalling and installing the module wasn't doing it for me, and I was still getting configuration errors. After a bit of investigation, I found out that the alt_flags config is expecting an array, when the default config value on install was set to string. When I changed the default value to an empty array, the configuration page resumed working immediately after reinstalling the module.

I also added a default composer.json file.