opentelemetry: Fix edge case

When no indexes key was present in the default values, we were forced to systematically
declare the index names (which was not initially intended). Adding this key make it
optional as intended (default values are computed in the chart).

Refs. swh/infra/sysadm-environment#4524

cluster-configuration/values.yaml

@@ -46,7 +46,7 @@ otlpCollector:
   debug: false
   version: 0.57.0
   # namespace: opentelemetry
-  # indexes:
+  indexes: {}
   #   swh: staging-logs
   #   system: staging-system-logs
   # resources: