InitSshd: Use correct flag to set empty passphrase for all key types

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.

Change Iba3a674f8 did this for RSA keys, but not for other key types.

Also do it for the other types.

Change-Id: Ied6d8d97d2333a313a0abf215bbdba5e1bba51bc
diff --git a/java/com/google/gerrit/pgm/init/InitSshd.java b/java/com/google/gerrit/pgm/init/InitSshd.java
index 68bdefc..c0adbe7 100644
--- a/java/com/google/gerrit/pgm/init/InitSshd.java
+++ b/java/com/google/gerrit/pgm/init/InitSshd.java
@@ -124,7 +124,7 @@
                   "-q" /* quiet */,
                   "-t",
                   "ed25519",
-                  "-P",
+                  "-N",
                   emptyPassphraseArg,
                   "-C",
                   comment,
@@ -152,7 +152,7 @@
                   "ecdsa",
                   "-b",
                   "256",
-                  "-P",
+                  "-N",
                   emptyPassphraseArg,
                   "-C",
                   comment,
@@ -180,7 +180,7 @@
                   "ecdsa",
                   "-b",
                   "384",
-                  "-P",
+                  "-N",
                   emptyPassphraseArg,
                   "-C",
                   comment,
@@ -208,7 +208,7 @@
                   "ecdsa",
                   "-b",
                   "521",
-                  "-P",
+                  "-N",
                   emptyPassphraseArg,
                   "-C",
                   comment,