In FileAdmin, if a directory has no date, it should display as empty (not 1970)

[pamelafox]

Discussed in #2573

Currently, directories are given a timestamp of 0, so they render as 1970:

It would be more accurate to show their dates as empty, like Azure storage explorer does:

(It would be good to know what S3 does, I don't have a current account. But I think the S3 code in flask-admin also sets a timestamp of 0 for directories).

This would be a change to BaseFileAdmin and possibly the templates.