Fix error component without object on Rails main to display properly

Rails main changed `object` to be `false` instead of `nil`, causing the
specific check we were doing for `object.nil?` to fail, and the custom
errors were not displaying for forms without an object. Our test suite
caught that, and the fix is to simply not check specifically for `nil?`,
but just whether we have an object or not, which should account for both
`nil`/`false` versions.

It's a small backwards incompatibility, probably not something worth
changing the framework for, but one worth mentioning.

Source is likely to be:
- rails/rails#49943
- rails/rails#50931

lib/simple_form/components/errors.rb

@@ -11,7 +11,7 @@ def full_error(wrapper_options = nil)
       end
 
       def has_errors?
-        object_with_errors? || object.nil? && has_custom_error?
+        object_with_errors? || !object && has_custom_error?
       end
 
       def has_value?