Stop trying to chmod if SerializerPermissions is null, fixes #71

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

library/HTMLPurifier/ConfigSchema/schema/Cache.SerializerPermissions.txt

@@ -1,5 +1,5 @@
 Cache.SerializerPermissions
-TYPE: int
+TYPE: int/null
 VERSION: 4.3.0
 DEFAULT: 0755
 --DESCRIPTION--
@@ -8,4 +8,9 @@ DEFAULT: 0755
     Directory permissions of the files and directories created inside
     the DefinitionCache/Serializer or other custom serializer path.
 </p>
+<p>
+    In HTML Purifier 4.8.0, this also supports <code>NULL</code>,
+    which means that no chmod'ing or directory creation shall
+    occur.
+</p>
 --# vim: et sw=4 sts=4