FIX: Error not being raised for required typed categories property (#26443)

Why this change?

For a schema like this:

```
schema = {
  name: "section",
  properties: {
    category_property: {
      type: "categories",
      required: true,
    },
  },
}
```

When the value of the property is set to an empty array, we are not
raising an error which we should because the property is marked as
required.

lib/theme_settings_object_validator.rb

@@ -208,7 +208,7 @@ class ThemeSettingsObjectValidator
   end
 
   def is_property_present?(property_name)
-    if @object[property_name].nil?
+    if @object[property_name].blank?
       add_error(property_name, :required)
       false
     else