When throw was used in a controller action, and there is matching catch
around the request in a Rack middleware, then :exception won't be
present in the event payload.

This is because ActiveSupport::Notifications::Instrumenter.instrument
sets :exception in a rescue handler, but rescue is never called in a
throw/catch scenario:

  catch(:halt) do
    begin
      throw :halt
    rescue Exception => e
      puts "rescue" # never reached
    ensure
      puts "ensure"
    end
  end

Missing :exception was actually handled prior to Rails 6.1.0, but an
optimization updated the code to assume this was present. So this can be
considered a regression fix.