Bug 8391: prevent error when viewing circ history with bad dates

Bug 8391: prevent error when viewing circ history with bad dates

Sometime the circ history will contain "0000-00-00" as a returndate when
an item was lost rather than returned. This currently causes an error
when attempting to parse the dates, this patch causes an empty string to
be returned instead.

Note: a future enhancement should distinguish between "no date provided"
and "invalid date provided" to allow distinctions to be made.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>