chore: simplified variable check condition (#404)

Author: 0xkazak

nethermind/nethermind-entrypoint

@@ -25,7 +25,7 @@ fi
 mkdir -p "$NETHERMIND_DATA_DIR"
 
 # Write the JWT secret
-if [[ -z "${OP_NODE_L2_ENGINE_AUTH_RAW:-}" ]]; then
+if [[ -z "$OP_NODE_L2_ENGINE_AUTH_RAW" ]]; then
     echo "Expected OP_NODE_L2_ENGINE_AUTH_RAW to be set" 1>&2
     exit 1
 fi