Invalid datetime params from flag series requests

[aliu39]

SENTRY-3GZJ
The flag series component is used by issues' event graph to show vertical lines when a flag was updated. It gets its data from OrganizationFlagLogIndex endpoint. It's making a lot of requests where the time range has start=end. Both are ISO strings w/second precision.

To resolve this we should

1. investigate when/why we pass in start=end
2. stop making these requests (just set response=empty)