fix(dv-pod): default fetchFeeRecipientUpdates to true when nil

[nickh-obol]

Summary

• helm upgrade --reuse-values from a chart predating the charon.fetchFeeRecipientUpdates key (e.g. dv-pod-0.17.0/0.18.x) carries forward the previous release's effective values without merging the upgraded chart's new values.yaml defaults. The template then resolved .Values.charon.fetchFeeRecipientUpdates to nil and rendered --fetch-feerecipient-updates= (empty), and charon refused to start with invalid argument "" for "--fetch-feerecipient-updates" flag: strconv.ParseBool: parsing "": invalid syntax.
• Fix renders true when the value is nil while preserving an explicit true or false from the user. Keeps PR feat(dv-pod): add fetchFeeRecipientUpdates charon flag #272's "always render the flag" intent so charon's own false default never silently takes over.
• Mitigates the issue for any operator who upgrades with --reuse-values (the long-standing default many docs and habits assume) without having to know about --reset-then-reuse-values.

Test plan

• helm template charts/dv-pod --set charon.fetchFeeRecipientUpdates=null → renders --fetch-feerecipient-updates=true
• helm template charts/dv-pod --set charon.fetchFeeRecipientUpdates=true → renders --fetch-feerecipient-updates=true
• helm template charts/dv-pod --set charon.fetchFeeRecipientUpdates=false → renders --fetch-feerecipient-updates=false
• helm template charts/dv-pod (default) → renders --fetch-feerecipient-updates=true
• helm lint charts/dv-pod passes
• make docs produces no diff (template-only change)
• Reviewer confirms Auto Version Bump workflow ticks chart 0.19.0 → 0.19.1 (patch — appVersion unchanged)