ZTS: Fix summary page creation

There are cases, where some needed files for the summary page aren't
created. Currently the whole Summary Page creation will fail then.
Sample run: https://github.com/openzfs/zfs/actions/runs/11148248072/job/30999748588

Fix this, by properly checking for existence of the needed files.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #16599