Fix rules_expression.* configuration schema

The schema for rules_expression is wrong (found in rules.expression.schema.yml) - it is defined as a type: mapping but there is no mapping given. Because of this, all the other schemas that inherit from the rules_expression type end up defining their own id: and uuid: elements - this should not be necessary because all the rules_expression.* schemas should inherit these elements from the parent rules_expression.

Patch fixes this. Manually tested this using the config_inspector module and all configured Rules on my site still pass schema validation after the patch.

NOTE: This patch is a prelude to fixing #2648334: Implement action/condition reordering with drag & drop because we can't add a 'weight' element that is inherited by all expressions unless the schemas are properly defined.