Install v5.0.3-3 ignoring existingSecret for postgres

[Ishiguro872]

Tried installing with using an existingSecret in values.yaml:

helm upgrade listmonk listmonk \
  --create-namespace \
  --install \
  --namespace listmonk \
  --repo https://th0th.github.io/helm-charts \
  --values values.yaml \
  --version 5.0.3-3

fails with:

Release "listmonk" does not exist. Installing it now.
Error: execution error at (listmonk/templates/secret-postgres.yaml:9:15): A valid .Values.postgres.password is required

[th0th]

@libo can you please have a look at this? It seems to be related to #20.