Support setting dataSourceConfig through secrets or extraEnv

[philipclaesson]

This PR contains two changes which are necessary in order to support passing dataSourceConfig as an env variable (through extraEnv or secrets) rather than via values.yaml.

1. Don't set env variable OPAL_DATA_CONFIG_SOURCES if config: null
2. Don't set env variable OPAL_DATA_UPDATER_ENABLED if it is passed as extraEnv

Before this change, running template with an empty datasources object

dataConfigSources:
	config: null

and also passing an extraEnv containing OPAL_DATA_UPDATER_ENABLED

extraEnv: {
"OPAL_DATA_UPDATER_ENABLED": "true"
}

would yield two conflicting definitions of the OPAL_DATA_UPDATER_ENABLED:

➜ helm template . -f values.yaml | grep DATA_UPDATER -A 1
            - name: OPAL_DATA_UPDATER_ENABLED
              value: "False"
--
            - name: OPAL_DATA_UPDATER_ENABLED
              value: "true"

After this change, the value passed in extraEnv takes precedence

➜ helm template . -f values.yaml | grep DATA_UPDATER -A 1
            - name: OPAL_DATA_UPDATER_ENABLED
              value: "true"

Furthermore, with config: null we do not set OPAL_DATA_CONFIG_SOURCES, and assume that it will be passed through extraEnv or secret.