Fix themes_config_file_by_host to always contain a dict

[ksuderman]

When themes are disabled, either by setting galaxy_manage_themes: false or if there are no galaxy_themes_subdomains, the use of the YAML folded block scalar operator (>) causes the variable themes_config_file_by_host to evaluate the string '{ }' rather than to an empty dictionary. This causes Galaxy's config parser to fail when it tries to call .items() on a string.

With this PR:

1. When themes are disabled (galaxy_manage_themes: false or no galaxy_themes_subdomains):
   - Returns an empty dict {}
   - Not a string "{ }"
2. When themes are enabled (both conditions true):
   - Uses _themes_config_file_by_host which is a proper YAML dictionary
   - Each host is a key with its corresponding theme config file as the value

Closes #231