Fix NoMethodError when sending is not allowed

[st0012]

Here's a full description of the issue #1147 (comment)

Stacktraces copied from the comment:

     NoMethodError:
       undefined method `event_id' for false:FalseClass
     Shared Example Group: "an unauthorized response" called from ./spec/requests/webhooks/slack_spec.rb:390
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/sentry-ruby-4.1.0/lib/sentry/hub.rb:113:in `capture_event'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/sentry-ruby-4.1.0/lib/sentry/hub.rb:85:in `capture_exception'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/sentry-ruby-4.1.0/lib/sentry-ruby.rb:119:in `capture_exception'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/sentry-ruby-4.1.0/lib/sentry/rack/capture_exceptions.rb:28:in `rescue in block in call'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/sentry-ruby-4.1.0/lib/sentry/rack/capture_exceptions.rb:22:in `block in call'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/sentry-ruby-4.1.0/lib/sentry/hub.rb:50:in `with_scope'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/sentry-ruby-4.1.0/lib/sentry-ruby.rb:103:in `with_scope'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/sentry-ruby-4.1.0/lib/sentry/rack/capture_exceptions.rb:14:in `call'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/rack-cors-1.1.1/lib/rack/cors.rb:100:in `call'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/railties-6.0.3.4/lib/rails/engine.rb:527:in `call'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/rack-test-1.1.0/lib/rack/mock_session.rb:29:in `request'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/rack-test-1.1.0/lib/rack/test.rb:266:in `process_request'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/rack-test-1.1.0/lib/rack/test.rb:119:in `request'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/actionpack-6.0.3.4/lib/action_dispatch/testing/integration.rb:270:in `process'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/actionpack-6.0.3.4/lib/action_dispatch/testing/integration.rb:24:in `post'
     # /usr/local/var/rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/actionpack-6.0.3.4/lib/action_dispatch/testing/integration.rb:359:in `block (2 levels) in <module:Runner>'