Cube's last modified time will set to wrong date when incremental job failed

[lukehan]

When incremental refresh job failed, the cube's last modified time will set to 1970-01-01 08:00:00
This is wrong value, it should always keep the last success refresh timestamp.

Screenshot:

[janzhongi]

when the last segment is null.system does not show last_modified_time correctly.
has fixed this issue.
the last_build_time will be the latest avaliable 'Ready' status segment.
if no segment,will show cre_time by default.