Allow testnet dir without boot nodes

beacon_node/src/config.rs

@@ -350,11 +350,12 @@ fn init_new_client<E: EthSpec>(
         .deposit_contract_deploy_block
         .saturating_sub(client_config.eth1.follow_distance * 2);
 
-    client_config.network.boot_nodes = eth2_testnet_dir
-        .boot_enr
-        .as_ref()
-        .ok_or_else(|| "Testnet directory does not contain any boot nodes")?
-        .clone();
+    if let Some(boot_nodes) = eth2_testnet_dir.boot_enr {
+        client_config
+            .network
+            .boot_nodes
+            .append(&mut boot_nodes.clone())
+    }
 
     if let Some(genesis_state) = eth2_testnet_dir.genesis_state {
         // Note: re-serializing the genesis state is not so efficient, however it avoids adding