Fix missing default scrapeInterval and evaluationInterval

[troytop]

What this PR does / why we need it

If you pass the default values.yaml to helm install without these two keys set, the secret in prometheus-prometheus-kube-prometheus-prometheus 😞 is set to an empty string, which is invalid and causes it to fail to start.

[jkroepke]

dont get this.

{{- if .Values.prometheus.prometheusSpec.scrapeInterval }}
  scrapeInterval: {{ .Values.prometheus.prometheusSpec.scrapeInterval }}
{{- end }}

scrapeInterval is covered by an if condition. If scrapeInterval is empty, the if condition is false and the block is skipped

[troytop]

Perhaps this is only an issue when the user takes the chart's values.yaml and edits it for use as their values file. In that case scrapeInterval and evaluationInterval are explicitly set to an empty string. This scenario happens in the default Helm install workflow in the Rancher UI, but it might be an edge case for normal helm usage.

[jkroepke]

Empty string is false, too.

If and default share the same
logic. There is no known condition where the if return true, but the default filter returns the default value.