FR: Allow Client to provide a zero-length ClientId

[Ryan-CW-Code]

According to the MQTT protocol, clients can transmit zero-length ClientId, but coreMqtt considers zero-length ClientId to be illegal

https://github.com/FreeRTOS/coreMQTT/blob/ad72f6aff6c08ac7fe91d7aac0ea6b046b52cd94/source/core_mqtt_serializer.c#L1709C16-L1709C53

[AniruddhaKanhere]

Hello @Ryan-CW-Code,

Thank you for taking the time to report this issue to us.

Looking at the screenshot and the code we have, it seems to me that you are correct.

However, let me take a deeper look and respond back with my assessment.

Thanks,
Aniruddha

[AniruddhaKanhere]

This commit added this check. Not entirely sure why was this done.

I can guess that this code was tested against AWS IoT core which does not support NULL client IDs as it uses client IDs to differentiate clients from one another. See here. But that is just a guess.

In any case, seems that we are deviating from the specification. We should add the check in.

@Ryan-CW-Code would you like to fix the code by raising a PR? If not, I will be happy to make that change and request your review.

Let me know what would you prefer.

Thanks again,
Aniruddha

[Ryan-CW-Code]

#330
Please review
If you have better ideas, please modify them. Thanks