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]

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