queuing_off() should give warning if the model doesn't have a queue

[apulsipher]

This pull request adds a warning to the queuing_off() function for ModelSIRCONN and ModelSEIRCONN classes, because they don't have a queue. This behavior mirrors the behavior of queuing_on() for these model classes.

Additionally, this pull request updates the test-sirconn.R file to cover the expected warnings

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 36.17%. Comparing base (174263f) to head (47b267b).

Files with missing lines	Patch %	Lines
R/model-methods.R	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
+ Coverage   36.15%   36.17%   +0.02%     
==========================================
  Files          36       36              
  Lines        1975     1979       +4     
==========================================
+ Hits          714      716       +2     
- Misses       1261     1263       +2     

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