Fix schema generation for c#9 positional record with param tag without example property creating unexpected empty example

[stb-co]

Pull Request

The issue or feature being addressed

Fixes #2900

Details on the issue fix or feature implementation

I added a check for the case where the example string would be emtpy due to not being found.
I also added a test for when the example property is not provided.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.89%. Comparing base (828b7df) to head (6815b8b).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2901   +/-   ##
=======================================
  Coverage   89.88%   89.89%           
=======================================
  Files          72       72           
  Lines        2710     2711    +1     
  Branches      428      429    +1     
=======================================
+ Hits         2436     2437    +1     
  Misses        274      274           

Flag	Coverage Δ
Linux	89.89% <100.00%> (+<0.01%)	⬆️
Windows	?
macOS	?

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