Allow empty sections in string hashes; previously they were left undefined.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

tests/HTMLPurifier/StringHashParserTest.php

@@ -29,6 +29,7 @@ class HTMLPurifier_StringHashParserTest extends UnitTestCase
             'TYPE' => 'string',
             'CHAIN-ME' => '2',
             'DESCRIPTION' => "Multiline\nstuff\n",
+            'EMPTY' => '',
             'FOR-WHO' => "Single multiline\n",
         ));
     }