chore(RBAC) disable session conf secret if not set

What this PR does / why we need it:

Replace the default placeholder value for enterprise.rbac.session_conf_secret with an empty string.

Disable configuring a session conf envvar if session_conf_secret is not set.

From https://docs.konghq.com/gateway/changelog/#3600 and https://docs.konghq.com/gateway/3.6.x/kong-manager/auth/oidc/migrate/, Kong 3.6+ uses only KONG_ADMIN_GUI_AUTH_CONF when using OIDC for Manager.

Prior to 3.6, Kong required KONG_ADMIN_GUI_SESSION_CONF when using RBAC. The original chart design effectively required users set session_conf_secret as it would try to reference a nonexistent placeholder Secret name otherwise.

Special notes for your reviewer:

Waiting for confirmation from the reporter that we don't actually need anything else to handle 3.6 properly.

Checklist

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

[x] PR is based off the current tip of the main branch.
[x] Changes are documented under the "Unreleased" header in CHANGELOG.md
[x] New or modified sections of values.yaml are documented in the README.md
[x] Commits follow the Kong commit message guidelines

Kong / charts