Did you find a way to manage this? I have the same issue.
Hello! Here in 2020, still seeing this issue, with my Dashboard widget. The workaround (adding an "is not Null" filter) of course works, but does anybody have information about this in a more general sense? Will Null date values always default to this date?
Null values showing a date of 12/31/1969:
After adding an "is not Null" filter: