Enhance application-x-struct data type

[christophd]

• Support empty message body value and initialize the application-x-struct data type with empty JsonNode {} value
• Allows follow-up action Kamelets such as insert-field-action to add more fields to the empty JsonNode body

Example:

1. webhook-source creates empty message body
2. application-x-struct data-type used as output data type initializes empty JsonNode
3. insert-field action Kamelet uses header fields to set fields on the JsonNode
4. Json struct gets serialized to String value as outcome

[christophd]

Webhook called with http://localhost:8080/bookings?name=foo&amount=100
result in Json {"name": "foo", "amount": "100"} on Kafka sink

apiVersion: camel.apache.org/v1alpha1
kind: KameletBinding
metadata:
  name: webhook-source-booking-pipe
spec:
  source:
    ref:
      kind: Kamelet
      apiVersion: camel.apache.org/v1
      name: webhook-source
    properties:
      subpath: bookings
    dataTypes:
      out:
        format: application-x-struct
  steps:
    - ref:
        kind: Kamelet
        apiVersion: camel.apache.org/v1
        name: insert-field-action
      properties:
        field: fruit
        value: ${header[name]}
    - ref:
        kind: Kamelet
        apiVersion: camel.apache.org/v1
        name: insert-field-action
      properties:
        field: amount
        value: ${header[amount]}
  sink:
    ref:
      kind: KafkaTopic
      apiVersion: kafka.strimzi.io/v1beta2
      name: bookings
    properties:
      brokers: localhost:9092
    dataTypes:
      in:
        format: plain-text