Properly handle '0' filenames in import-tars

Properly handle '0' filenames in import-tars

Randal L. Schwartz pointed out multiple times that we should be
testing the length of the name string here, not if it is "true".
The problem is the string '0' is actually false in Perl when we
try to evaluate it in this context, as '0' is 0 numerically and
the number 0 is treated as a false value.  This would cause us
to break out of the import loop early if anyone had a file or
directory named "0".

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>