Fix datetime functions to return correct results for 0 and false

[angelamayxie]

Fixes dolthub/dolt#10075

Our datetime functions were not returning the correct results for 0 and false. This was because we were using 0000-01-01 as zeroTime (which evaluates to true using Go's time.IsZero) and then extracting datetime information from that timestamp. Using 0000-01-01 as zeroTime was also giving incorrect results for some functions when the input timestamp was actually 0000-01-01, since it was being equated as zeroTime.

Also, depending on whether false was read as false or 0, we were also not able to convert it to zeroTime.

This fix involves updating zeroTime to time.Date(0, 0, 0, 0, 0, 0, 0, time.UTC), which has the timestamp -0001-11-30. Since negative years are not valid in MySQL, this timestamp would not conflict with a non-zero timestamp. We also add in checks for zeroTime to make sure functions return the correct value from zeroTime instead of simply extracting datetime information from the timestamp.

Dolt bump PR: dolthub/dolt#10084

[jycor]

LGTM