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