TTFD timeout timer never scheduled

[armcknight]

Description

A customer reported that if they never call reportFullyDisplayed, they don't get a transaction. The headerdoc for enableTimeToFullDisplayTracing state:

 * If @c SentrySDK.reportFullyDisplayed() is not called, the transaction will finish
 * automatically after 30 seconds and the `Time to full display` Span will be
 * finished with @c DeadlineExceeded status.

But I was able to repro this, and it turns out that when scheduling the timeout timer for the span in startDeadlineTimer, _configuration.dispatchQueueWrapper is nil so no deadline timer ever fires.