InitSshd: Use correct flag to set empty passphrase

From https://www.ssh.com/ssh/keygen/:
  -N "New" Provides a new passphrase for the key.
  -P "Passphrase" Provides the (old) passphrase when reading a key.

PiperOrigin-RevId: 210948400
Change-Id: Iba3a674f81686987fef60f0fd5a5f42aa5b9dd86

java/com/google/gerrit/pgm/init/InitSshd.java

@@ -103,7 +103,7 @@ public class InitSshd implements InitStep {
                 "-q" /* quiet */,
                 "-t",
                 "rsa",
-                "-P",
+                "-N",
                 emptyPassphraseArg,
                 "-C",
                 comment,