core: Error when return a json as payload_template in automation actions

The problem

 action:
    service: mqtt.publish
    data_template:
      topic: "topic/test"
      payload_template: >
        {% set payload = {'value1': 'test', 'value2': 'test' } %}
        {{ payload|to_json }}
      qos: 0
      retain: true

Environment

• Home Assistant Core release with the issue: 0.117.0b1
• Last working Home Assistant Core release (if known): 0.116

Traceback/Error logs

Error executing script. Invalid data for call_service at pos 1: value should be a string for dictionary value @ data['payload_template']