ZTS: Fix summary page creation

[mcmilk]

Motivation and Context

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.

How Has This Been Tested?

The ZTS will do.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[robn]

Two earnest and dumb questions:

• is a missing file interesting? Would it be better to put [uname.txt not found] in the report instead?
• -s will skip zero-byte files too. Should it be -e? and/or, put [uname.txt empty] in the report?

[mcmilk]

Two earnest and dumb questions:

• is a missing file interesting? Would it be better to put [uname.txt not found] in the report instead?

No, we have the red light that indicates that e.g. the module build has failed.

• -s will skip zero-byte files too. Should it be -e? and/or, put [uname.txt empty] in the report?

When uname.txt exists, then there should be some content in it, thats why I used -s :)

[robn]

Told you they were dumb 😊