Fix error events missing a DSC when there's an active span

[sl0thentr0py]

We were actually missing the DSC in the envelope header in the case when we have a non-transaction event (like an error) being sent but with an active span/transaction on the scope.

fixes #2400

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (c3bcfa0) to head (57e979b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2408      +/-   ##
==========================================
- Coverage   98.69%   98.68%   -0.01%     
==========================================
  Files         210      210              
  Lines       13922    13935      +13     
==========================================
+ Hits        13740    13752      +12     
- Misses        182      183       +1     

Components	Coverage Δ
sentry-ruby	99.05% <100.00%> (-0.01%)	⬇️
sentry-rails	97.34% <ø> (ø)
sentry-sidekiq	97.07% <ø> (ø)
sentry-resque	97.11% <ø> (ø)
sentry-delayed_job	98.92% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files with missing lines	Coverage Δ
sentry-ruby/lib/sentry/propagation_context.rb	100.00% <ø> (ø)
sentry-ruby/lib/sentry/scope.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/span.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/scope_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/span_spec.rb	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes