The Discord template printed `value: {{ .ValueString }}` unconditionally, so a
notification carrying no value rendered as "value:" with nothing after it —
which reads as a value that failed to compute rather than one that was never
applicable.
ValueString is only populated when the notification came from an evaluation
that produced refIds. An instance resolved by Grafana's staleness handling has
none: no evaluation returns it again, so the state manager expires it. That is
triggered by a rule title change, since the title becomes the alertname label
and the old label set is orphaned — renaming ProdWorkerContainerSilent to
ProdWorkerHeartbeatMissing did it and posted exactly that message.
Guarding the whole line rather than the substitution: an unguarded
{{ .ValueString }} is harmless on its own, printing the label is the bug.
Verified by executing both templates through text/template with ValueString
set and empty: parses and executes clean, populated output is byte-identical
to the previous format, empty case omits the line.
