Check if ActiveRecord connection exists before calling AR connection pool

[st0012]

Fixes #1745

[codecov-commenter]

Codecov Report

Merging #1769 (f02936b) into master (4352445) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1769      +/-   ##
==========================================
+ Coverage   98.44%   98.45%   +0.01%     
==========================================
  Files         144      144              
  Lines        8439     8441       +2     
==========================================
+ Hits         8308     8311       +3     
+ Misses        131      130       -1     

Impacted Files	Coverage Δ
sentry-rails/lib/sentry/rails/background_worker.rb	85.71% <100.00%> (-14.29%)	⬇️
sentry-ruby/lib/sentry/background_worker.rb	100.00% <0.00%> (+5.40%)	⬆️

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