fix: Read and send unfinished spans

[bitsandfoxes]

Fixes getsentry/sentry-unity#1759

With getsentry/relay#1244 spans no longer need to be finished before sent. Without making reading the endTimestamp optional we end up with an error

System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.

[jamescrosswell]

LGTM

[github-actions]

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