reth-entrypoint: use safe parameter expansion to prevent set -u unbound variable errors

[Snezhkko]

Reason: With set -u, referencing unset variables in conditions (e.g., [[ -z "$RETH_CHAIN" ]] and checking RETH_FB_WEBSOCKET_URL) causes an “unbound variable” error, terminating the script unexpectedly.
Change: Updated conditional checks to use safe expansion ${RETH_CHAIN:-} and ${RETH_FB_WEBSOCKET_URL:-}.
Goal: Prevent crashes when optional env vars are unset while preserving existing behavior when they are provided.

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[danyalprout]

lgtm, thank you

[danyalprout]

lgtm, thank you

[Tuniex01]

Check