system: do not prompt for password on user edit

PR: https://lists.opnsense.org/pipermail/users/2017-March/000062.html
(cherry picked from commit a2381e6c)

src/www/system_usermanager.php

@@ -235,10 +235,6 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
             $input_errors[] = gettext('Cannot set random password due to explicit input.');
         }
 
-        if (empty($pconfig['passwordfld1']) && empty($pconfig['gen_new_password'])) {
-            $input_errors[] = gettext('A password is required.');
-        }
-
         if (!empty($pconfig['disabled']) && $_SESSION['Username'] === $a_user[$id]['name']) {
             $input_errors[] = gettext('You cannot disable yourself.');
         }
@@ -246,8 +242,13 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
         if (isset($id)) {
             $oldusername = $a_user[$id]['name'];
         } else {
-            $oldusername = "";
+            $oldusername = '';
+
+            if (empty($pconfig['passwordfld1']) && empty($pconfig['gen_new_password'])) {
+                $input_errors[] = gettext('A password is required.');
+            }
         }
+
         /* make sure this user name is unique */
         if (count($input_errors) == 0) {
             foreach ($a_user as $userent) {
@@ -257,6 +258,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
                 }
             }
         }
+
         /* also make sure it is not reserved */
         if (count($input_errors) == 0) {
             $system_users = explode("\n", file_get_contents("/etc/passwd"));