Make --file optional for generate-node-key (#7043)

This pr makes the `--file` argument optional to `generate-node-key`.
If the argument is not given, the secret node key will be printed to
`stdout`. The public node key will always be printed to `stderr`.

substrate/bin/utils/subkey/src/lib.rs

@@ -31,7 +31,8 @@ use sp_core::crypto::Ss58Codec;
 	about = "Utility for generating and restoring with Substrate keys",
 )]
 pub enum Subkey {
-	/// Generate a random node libp2p key, save it to file and print its peer ID
+	/// Generate a random node libp2p key, save it to file or print it to stdout
+	/// and print its peer ID to stderr.
 	GenerateNodeKey(GenerateNodeKeyCmd),
 
 	/// Generate a random account