MqttMessageBuilders builds invalid connect messages that `MqttEncoder.INSTANCE` seems to drop

Expected behavior

Trying to construct a connect message without a clientId should throw an exception,
as per http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718031

The Client Identifier (ClientId) MUST be present and MUST be the first field in the CONNECT packet payload[MQTT-3.1.3-3].

Actual behavior

instead the builder builds it, and the message never makes it through the pipeline, it seems to get dropped by MqttEncoder.INSTANCE without any kind of useful error message (even with TRACE level logging)

Steps to reproduce

Code to create the connect message

    MqttMessageBuilders
      .connect()
      .username(Base64.getEncoder.encodeToString(deviceID.toBytes))
// Don't include this line
//      .clientId("a-client-id")
      .build()

pipeline code

            ch.pipeline().addLast(new LoggingHandler(LogLevel.TRACE)) // For debugging
            ch.pipeline().addLast("mqttDecoder", new MqttDecoder())
//             This thing silently steals the connect message when we don't have client ID
            ch.pipeline().addLast("mqttEncoder", MqttEncoder.INSTANCE)
            ch.pipeline().addLast("DFEHandler", new OurHandlerThatProducesConnectMessages(config, logger))

Minimal yet complete reproducer code (or URL to code)

Netty version

4.1.73.Final

JVM version (e.g. java -version)

openjdk version "11.0.11" 2021-04-20
OpenJDK Runtime Environment AdoptOpenJDK-11.0.11+9 (build 11.0.11+9)
OpenJDK 64-Bit Server VM AdoptOpenJDK-11.0.11+9 (build 11.0.11+9, mixed mode)

OS version (e.g. uname -a)

21.6.0 Darwin Kernel Version 21.6.0

Start by tracing MqttMessageBuilders.connect() through MqttEncoder.INSTANCE using the provided pipeline and Netty 4.1.73.Final. Reproduce the missing-clientId case and inspect how the CONNECT payload is handled. Done means the invalid message produces a useful exception or error rather than being silently dropped, with coverage for the missing clientId behavior.