Make some Mexc.API.V1.Contract.TickerData fields nullable

[IlyaVshivtsev]

Pull request checklist

• Did you bump the project version?
• Did you add a description to the Pull Request?
• Did you add new tests?
• Did you add reviewers?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release/v0.35.0@346489c). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.35.0     #89   +/-   ##
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  Coverage                   ?   4.77%           
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  Files                      ?     316           
  Lines                      ?    2931           
  Branches                   ?       0           
=================================================
  Hits                       ?     140           
  Misses                     ?    2791           
  Partials                   ?       0           

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