fix bug introduced by #2655 (#2711)

# What this PR does
fixes a bug introduced in #2655 where the `topologySpreadConstraints`
helm values are not scoped correctly on the engine template.

## Which issue(s) this PR fixes
closes #2655 

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

Co-authored-by: Joey Orlando <joey.orlando@grafana.com>

helm/oncall/templates/engine/deployment.yaml

@@ -96,7 +96,7 @@ spec:
       tolerations:
         {{- toYaml . | nindent 8 }}
       {{- end }}
-      {{- if .Values.engine.topologySpreadConstraints }}
+      {{- with .Values.engine.topologySpreadConstraints }}
       topologySpreadConstraints:
         {{- toYaml . | nindent 8 }}
       {{- end }}