core.idl change to default Data frame payload to null

[cmebarrow]

Allows Data frames to have null payload (as distinct from zero length payload).

This is required for KTable support in the Kafka nukleus, where we need to be able to represent messages with value null.

Related PRs uptaking this change:

• https://github.com/reaktivity/nukleus-maven-plugin/pull/56
  • k3po-nukleus-ext: https://github.com/reaktivity/k3po-nukleus-ext.java/pull/33
  • tcp.spec: https://github.com/reaktivity/nukleus-tcp.spec/pull/27
  • tls.spec: https://github.com/reaktivity/nukleus-tls.spec/pull/12
  • http.spec: https://github.com/reaktivity/nukleus-http.spec/pull/45
  • http2.spec: https://github.com/reaktivity/nukleus-http2.spec/pull/42
  • ws.spec: https://github.com/reaktivity/nukleus-ws.spec/pull/21
  • sse.spec: https://github.com/reaktivity/nukleus-sse.spec/pull/4
  • http-cache.spec: https://github.com/reaktivity/nukleus-http-cache.spec/pull/49
  • nukleus-auth-jwt.spec: https://github.com/reaktivity/nukleus-auth-jwt.spec/pull/5
  • kafka.spec: https://github.com/reaktivity/nukleus-kafka.spec/pull/7
  • kafka-sse.spec: https://github.com/kaazing/nukleus-kafka-sse.spec/pull/8
  • command.log: https://github.com/reaktivity/command-log.java/pull/15
  • reaktor.java: https://github.com/reaktivity/reaktor.java/pull/27
    • tcp.java: https://github.com/reaktivity/nukleus-tcp.java/pull/58
    • tls.java: https://github.com/reaktivity/nukleus-tls.java/pull/27
    • http.java: https://github.com/reaktivity/nukleus-http.java/pull/63
    • http2.java: https://github.com/reaktivity/nukleus-http2.java/pull/82
    • ws.java: https://github.com/reaktivity/nukleus-ws.java/pull/33
    • sse.java: https://github.com/reaktivity/nukleus-sse.java/pull/7
    • http-cache.java: https://github.com/reaktivity/nukleus-http-cache.java/pull/70
    • nukleus-auth-jwt.java: https://github.com/reaktivity/nukleus-auth-jwt.java/pull/8
    • kafka.java: https://github.com/reaktivity/nukleus-kafka.java/pull/16
    • kafka-sse.java: https://github.com/kaazing/nukleus-kafka-sse.java/pull/13