Mixed up my Google search a little and found the answer in the Refinery Google Group:
https://groups.google.com/forum/#!msg/refinery-cms/0FfuehWwLgA/kuCgLVlf-nsJ
To fix this:
1. Add a decorator file for your admin page controller at path:
app/decorators/controllers/refinery/admin/pages_controller_decorator.rb
2. Copy-paste the following code:
Refinery::Admin::PagesController.class_eval do
# Solves error when creating pages without title
# See: http://stackoverflow.com/q/19013244/1093087
before_filter :load_valid_templates, :only => [:create, :update, :edit, :new]
end
I now get an error message saying I need to fill in the title as expected.
Thanks to pascal huynh at the groups.google.com link above for the quick fix.