MDL-14072 - Backported from HEAD. Prevent NULLs, empties and numbers to be processed by:

- backup_encode_absolute_links() at backup
- restore_decode_absolute_links() at restore

backup/backuplib.php

@@ -1778,6 +1778,16 @@
 
         global $CFG,$preferences;
 
+    /// MDL-14072: Prevent NULLs, empties and numbers to be processed by the
+    /// heavy interlinking. Just a few cpu cycles saved.
+        if ($content === NULL) {
+            return NULL;
+        } else if ($content === '') {
+            return '';
+        } else if (is_numeric($content)) {
+            return $content;
+        }
+
         //Use one static variable to cache all the require_once calls that,
         //under PHP5 seems to increase load too much, and we are requiring
         //them here thousands of times (one per content). MDL-8700. 

backup/restorelib.php

@@ -2607,8 +2607,18 @@
     //step in the restore execution, because we need to have it 
     //finished to know all the oldid, newid equivaleces
     function restore_decode_absolute_links($content) {
-                                     
-        global $CFG,$restore;    
+
+        global $CFG,$restore;
+
+    /// MDL-14072: Prevent NULLs, empties and numbers to be processed by the
+    /// heavy interlinking. Just a few cpu cycles saved.
+        if ($content === NULL) {
+            return NULL;
+        } else if ($content === '') {
+            return '';
+        } else if (is_numeric($content)) {
+            return $content;
+        }
 
         //Now decode wwwroot and file.php calls
         $search = array ("$@FILEPHP@$");