fix: CacheLoader causes crash when no cache is used

[kuznetsss]

If cache is disabled or Clio starts with and empty DB, loader_ inside cache is not created. So calling CacheLoader::stop() or CacheLoader::wait() was causing crash.

[godexsoft]

Nice! Thanks for fixing this 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.63%. Comparing base (540e938) to head (ed81529).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1853      +/-   ##
===========================================
+ Coverage    71.61%   71.63%   +0.02%     
===========================================
  Files          330      330              
  Lines        13411    13413       +2     
  Branches      6824     6826       +2     
===========================================
+ Hits          9604     9609       +5     
+ Misses        1923     1920       -3     
  Partials      1884     1884              

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