ZTS: Fix Summary Page

The qemu-9-summary-page.sh script reads the file env.txt in the
first lines. When the module didn't build, this file was not copied
into the tarfile - causing the scipt to abort.

Fix: copy needed files into the tarfile in case of module build
failures. The fix ignores also empty tarfiles in future.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #16555