ifcopenshell.template: fix timestring ignoring an explicit timestamp of 0

create(timestamp=0) computed the FILE_NAME timestring with
`d.get("timestamp") or time.time()`, which treats 0 (a legitimate
epoch timestamp) as unset because 0 is falsy. The header ended up
with the current wall-clock time in FILE_NAME while IFCOWNERHISTORY
correctly stored CreationDate=0, an inconsistent pair of dates in
the same file. Switched to an explicit None check so an explicit
timestamp of 0 is honoured the same way any other explicit
timestamp is.

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