scheduler: fix panic in render_templates destructive update check

In #18054 we introduced a new field `render_templates` in the `restart`
block. Previously changes to the `restart` block were always non-destructive in
the scheduler but we now need to check the new field so that we can update the
template runner. The check assumed that the block was always non-nil, which
causes panics in our scheduler tests.

scheduler/util.go

@@ -260,8 +260,8 @@ func tasksUpdated(jobA, jobB *structs.Job, taskGroup string) comparison {
 
 	// Check if restart.render_templates is updated
 	// this requires a destructive update for template hook to receive the new config
-	if a.RestartPolicy.RenderTemplates != b.RestartPolicy.RenderTemplates {
-		return difference("group restart render_templates", a.RestartPolicy.RenderTemplates, b.RestartPolicy.RenderTemplates)
+	if c := renderTemplatesUpdated(a.RestartPolicy, b.RestartPolicy); c.modified {
+		return c
 	}
 
 	// Check each task
@@ -574,6 +574,23 @@ func spreadsUpdated(jobA, jobB *structs.Job, taskGroup string) comparison {
 	return same
 }
 
+// renderTemplatesUpdated returns the difference in the RestartPolicy's
+// render_templates field, if set
+func renderTemplatesUpdated(a, b *structs.RestartPolicy) comparison {
+
+	noRenderA := a == nil || !a.RenderTemplates
+	noRenderB := b == nil || !b.RenderTemplates
+
+	if noRenderA && !noRenderB {
+		return difference("group restart render_templates", false, true)
+	}
+	if !noRenderA && noRenderB {
+		return difference("group restart render_templates", true, false)
+	}
+
+	return same // both nil, or one nil and the other false
+}
+
 // setStatus is used to update the status of the evaluation
 func setStatus(logger log.Logger, planner Planner,
 	eval, nextEval, spawnedBlocked *structs.Evaluation,