Unit test find_min_max_timestamps_from_intervals()

[DanilYachmenev]

There is a find_min_max_timestamps_from_intervals() in dataflow/core/utils.py that is being used in HistoricalDataSource

We need to add unit tests to dataflow/core/test/test_utils.py.

Implement full coverage of the function but not too much.
I'd expect 5-7 small unit test methods that cover each if-else clause in the function body.

For in each method, try different value types - for this please understand Intervals value type beforehand (see)

See unit test doc to follow the code style

Assigning @samarth9008 to re-distribute
Could be a good 2nd task for those who already understand our approach to unit testing, here some thinking is required

FYI @gpsaggese @DanilYachmenev