TimestampChecker.Value returned a sentinel for "no source found": first
the string "0", then time.Unix(0, 0) once the value became a time.Time
exposed to templates. Rendered, that sentinel reads as a genuine date,
1970-01-01 01:00:00 +0100 CET, whose zone depends on the machine — so a
command interpolating {{.TIMESTAMP}} gets a plausible but meaningless
multi-word value instead of nothing.
Return an empty string, like NoneChecker already does for a checker with
nothing to report.