Actually return null on missing users

Several objects with getUser() functions (document, packet, etc) were expecting a null user if the ID wasn't found, but a UserNotFoundException was being thrown instead which prevented some index pages from being displayed.

Adds User::findUserById($id) function to libraries\User. Returns the user object or null. Updates some objects to use this.

Author: Davnit

src/libraries/Document.php

@@ -265,8 +265,7 @@ public function getURI() {
   }
 
   public function getUser() {
-    if (is_null($this->user_id)) return null;
-    return new User($this->user_id);
+    return User::findUserById($this->user_id);
   }
 
   public function getUserId() {

src/libraries/NewsPost.php

@@ -302,8 +302,7 @@ public function getURI() {
   }
 
   public function getUser() {
-    if (is_null($this->user_id)) return null;
-    return new User($this->user_id);
+    return User::findUserById($this->user_id);
   }
 
   public function getUserId() {

src/libraries/Packet.php

@@ -523,11 +523,7 @@ public function getUsedBy() {
   }
 
   public function getUser() {
-    if ( is_null( $this->user_id )) {
-      return null;
-    }
-
-    return new User( $this->user_id );
+    return User::findUserById($this->user_id);
   }
 
   public function getUserId() {

src/libraries/Server.php

@@ -177,8 +177,7 @@ public function getUpdatedDateTime() {
   }
 
   public function getUser() {
-    if (is_null($this->user_id)) return null;
-    return new User($this->user_id);
+    return User::findUserById($this->user_id);
   }
 
   public function getUserId() {

src/libraries/User.php

@@ -341,6 +341,16 @@ public function getAcl($acl) {
     return ($this->options_bitmask & $acl);
   }
 
+  public static function findUserById($user_id) {
+    if (is_null($user_id)) return null;
+    
+    try {
+      return new User($user_id);
+    } catch (UserNotFoundException $e) {
+      return null;
+    }
+  }
+  
   public static function &getAllUsers(
     $order = null, $limit = null, $index = null
   ) {