Prevent Iteration burndown charts trend line going backwards in time

What does this MR do and why?

Fix iteration burndown lines sometimes going backwards.
The data we fetch is sparse, then we pad empty days with the values from the previous day (as no data means no events for that day).
If we were missing data from the second last day, and a timezone was pre-UTC, then we were incorrectly checking "has data for last day" for the wrong day.

Testing

1. From the project sidebar, Manage > Iterations, then select an Iteration from the Done list (so due date is in the past)
2. In devtools, click the 3 dots menu, More Tools > Sensors, and set a timezone that is before UTC
3. Check that line doesn't go backwards. Or for iterations with no events, confirm the line goes all the way to the end

Screenshots or screen recordings

before	after

MR acceptance checklist

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Related to #383607 (closed)