Impossible to make a rules_expression.[id] nullable

Problem/Motivation

I have a config entity which has an optional expression. I tried to do this using the following config schema:

my_module.config_entity_type.*:
  type: config_entity
  mapping:
    expression:
      type: rules_expression.[id]
      label: 'Expression configuration'
      nullable: true

This doesn't work, because if the value is NULL, there is no ID and so it can't find a data type, resulting in schema errors and failing tests.

Proposed resolution

Defining a catch-all for rules_expression.* solves this.

Data model changes

Additional catch all schema to allow nulls.