Raise error on null mqtt5 client

[xiazhvera]

Issue #, if available:

Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 46.15385% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 82.56%. Comparing base (ed7bbd6) to head (5a78a56).

Files	Patch %	Lines
source/v5/mqtt5_client.c	46.15%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #369      +/-   ##
==========================================
- Coverage   82.62%   82.56%   -0.07%     
==========================================
  Files          20       20              
  Lines        8758     8768      +10     
==========================================
+ Hits         7236     7239       +3     
- Misses       1522     1529       +7     

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[sfod]

This PR relaxes the requirements for mqtt5 client to be valid, for some operations. I'm trying to figure out if this could potentially cause something bad in operations that don't validate client (e.g. https://github.com/awslabs/aws-c-mqtt/blob/v0.10.4/source/v5/mqtt5_client.c#L2109). Maybe I'm being too paranoid.

[xiazhvera]

The PR was to workaround the swift error issue. As there are some concern about adding the error in c-mqtt, I will re-evaluate the solution. Temporary close the pr.