Use "undef" instead of empty string when calling...

Use "undef" instead of empty string when calling Sympa::User::password_fingerprint without a previous password hash

src/bin/upgrade_sympa_password.pl.in

@@ -92,7 +92,7 @@ while (my $user = $sth->fetchrow_hashref('NAME_lc')) {
             q{UPDATE user_table
               SET password_user = ?
               WHERE email_user = ?},
-            Sympa::User::password_fingerprint($clear_password, ''),
+            Sympa::User::password_fingerprint($clear_password, undef),
             $user->{'email_user'}
         )
         ) {

src/lib/Sympa/User.pm

@@ -564,7 +564,7 @@ sub update_global_user {
     ## use hash fingerprint to store password
     ## hashes that use salts will randomly generate one
     $values->{'password'} =
-        Sympa::User::password_fingerprint($values->{'password'},'')
+        Sympa::User::password_fingerprint($values->{'password'}, undef)
         if ($values->{'password'});
 
     ## Canonicalize lang if possible.
@@ -642,9 +642,9 @@ sub add_global_user {
     my ($field, $value);
 
     ## encrypt password with the configured password hash algorithm
-    ## an empty salt means generate a new random one
+    ## an salt of 'undef' means generate a new random one
     $values->{'password'} =
-        Sympa::User::password_fingerprint($values->{'password'},'')
+        Sympa::User::password_fingerprint($values->{'password'}, undef)
         if ($values->{'password'});
 
     ## Canonicalize lang if possible