Sentry.with_child_span should check SDK's initialization state

[st0012]

Fixes #1818

[codecov-commenter]

Codecov Report

Merging #1819 (bc9dea5) into master (0fe8d27) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1819      +/-   ##
==========================================
+ Coverage   98.43%   98.45%   +0.01%     
==========================================
  Files         145      145              
  Lines        8587     8589       +2     
==========================================
+ Hits         8453     8456       +3     
+ Misses        134      133       -1     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry-ruby.rb	96.20% <100.00%> (-0.03%)	⬇️
sentry-ruby/spec/initialization_check_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/breadcrumb.rb	100.00% <0.00%> (+3.70%)	⬆️

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