fix: Check credentials secret for trivial values

[adwk67]

Description

Fixes #349.

The parent issues dates from 11/2023 and the behaviour seems to have changed since then. Deserialization errors are thrown if either clusterConfig is omitted entirely or credentialsSecret is omitted (or a value is missing). An error appears on the StatefulSet if only a quoted empty string is supplied.

With this PR, a deserialization error is thrown that catches both empty/missing cases:

Kind=AirflowCluster failed: failed to parse object: failed to deserialize object from JSON: Field cannot be empty or whitespace

However, I'm not really sure if this is an improvement as the name of the field is omitted in the message. Previously it was included :

AirflowCluster.airflow.stackable.tech "airflow" is invalid: spec.clusterConfig.credentialsSecret: Required value

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