Fix stats calculation for runs with missing start and end events
Previously, the event log for a run did not contain an event for PIPELINE_START or PIPELINE_SUCCESS | PIPELINE_FAILURE, then SqlEventLogStorage.get_stats_for_run would throw an error.
The error would be thrown because we convert start and end datetime object into floats, but in the case these events were missing, we would attempt to convert None.
This diff checks if the times exist, and then pass the datetime to date_time_as_float
Test Plan: unit
Reviewers: max, schrockn, alangenfeld
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D1726