searchidx: use Perl truthiness to detect XAPIAN_FLUSH_THRESHOLD

searchidx: use Perl truthiness to detect XAPIAN_FLUSH_THRESHOLD

XAPIAN_FLUSH_THRESHOLD is a C string in the environment, so
users may be tempted to assign an empty string in in their
shell, e.g. `XAPIAN_FLUSH_THRESHOLD= <command>' instead of using
`unset' POSIX shell built-in.

With either a value of "0" or "" (empty string), Xapian will
fall back to its default (10000 documents), which causes grief
for memory-starved users.