check for empty in disconnect reason bytes

[macfarla]

PR description

Fix bug introduced in #6925
UNKNOWN disconnect reason has null code

Fixed Issue(s)

Fixes #6937

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[shemnon]

Do we know what client is sending those?

[macfarla]

Do we know what client is sending those?

I think the one I saw was an outgoing disconnect. But we do have some logs to analyse, can get some more data on this.