[pgadmin4] fix bug where secret isnt created when using .serverDefinitions.resourceType

[alexhfmnn]

What this PR does / why we need it:

This PR is an addition to my previous one https://github.com/rowanruseler/helm-charts/pull/249. It fixes a bug, where the Secret manifest for Server Definitions isn't created, when .existingSecret is definied - which is usally the case when credentials are defined in a Secret. The combination of .serverDefinitions.resourceType: Secret and .serverDefinitions.servers: {firstServer: ...} now creates the corresponding secret.

Special notes for your reviewer:

Main issue is the (not .Values.existingSecret) here

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [X] DCO signed
• [X] Chart Version bumped
• [X] Variables are documented in the README.md
• [X] Title of the PR starts with chart name (e.g. [pgadmin4])