fix: use unset-safe expansions in entrypoint validation checks

[ygd58]

Problem

The entrypoint scripts use set -u which causes bash to exit when an unset variable is expanded. However, the validation logic uses -z "$VAR" which expands the variable before checking if it's empty.

This means if OP_NODE_NETWORK is unset and OP_NODE_ROLLUP_CONFIG is set (for op-node-entrypoint), the script crashes on unbound OP_NODE_NETWORK expansion instead of evaluating the condition correctly.

Fix

Use unset-safe expansions in checks (${VAR:-}) while keeping set -u for safety.

Changes

• op-node-entrypoint: Use ${OP_NODE_NETWORK:-} and ${OP_NODE_ROLLUP_CONFIG:-}
• nethermind/nethermind-entrypoint: Use ${OP_NODE_NETWORK:-} and ${OP_NODE_L2_ENGINE_AUTH_RAW:-}

Closes #989

[cb-heimdall]

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