[llvm-exegesis] Fix 'min' repetition mode in presence of missing measurements

[llvm-exegesis] Fix 'min' repetition mode in presence of missing measurements

This was a regression from 17e202424c021fd903950fec7a8b6cca2d83abce.
Previously we'd gracefully handle missing measurements,
but that handling got accidentally lost during the code move,
and we'd assert.

What we want to do, is to discard all measurements (from all repetitors
in a given config) if any of them failed, but do append the snippet,
and do emit the empty measurement.