Check if rate limiter is initialized before use

[Hayden-IO]

If the rate_limit flag is not specified, then the limiter will not be initialized. This will cause a panic at request time.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 23.02%. Comparing base (8df5d0f) to head (a6bd152).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
omniwitness/omniwitness.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #551   +/-   ##
=======================================
  Coverage   23.02%   23.02%           
=======================================
  Files          30       30           
  Lines        2063     2063           
=======================================
  Hits          475      475           
  Misses       1499     1499           
  Partials       89       89           

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[AlCutter]

Thanks!