[mbuckton]

Outbound MQTT 3.1.1 connections without credentials fail while packing CONNECT, and both outbound protocol implementations dereference a missing password when a username is supplied.

This change conditionally constructs and encodes credential fields, supports username-only connections, permits password-only connections in MQTT 5, and rejects password-only MQTT 3.1.1 connections before sending. MQTT 5 encoding distinguishes omitted credentials from explicitly empty values; MQTT 3.1.1 credential lengths use the encoded username byte count. Session identities, routing and authentication policy are unchanged. Existing password trimming remains unchanged.

Validation

• Added 30 focused credential tests; observed 20 failures before the fix and all 30 passing afterwards.
• Ran the new tests plus keepalive, session identity and MQTT 5 CONNECT regressions: 44 tests passed using JDK 25 and JUnit against cached dependencies, compiling the four changed production classes from source.
• git diff --check passed.
• Maven command mvn -o -Dtest=MQTTConnectCredentialsTest test stopped before compilation because the required io.mapsmessaging:license_manager:pom:3.3.7 descriptor was unavailable locally. A full Maven build, full suite and live server interoperability remain unverified.

Refs: MSG-421
https://mapsmessaging.atlassian.net/browse/MSG-421

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