Do not special case null for URLUtils attributes

LayoutTests/fast/dom/HTMLAnchorElement/set-href-attribute-hash-expected.txt

 Test setting the hash attribute of the URL in HTMLAnchorElement.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 Hash value does not start with '#'
 PASS a.href is 'https://www.mydomain.com:8080/path/testurl.html#hash-value'
 Hash value starts with '#'
 PASS a.href is 'file:///path/testurl.html#hash_value'
 '?' in hash value
 PASS a.href is 'http://www.mydomain.com/path/testurl.html#hash?value'
 '#' in hash value, and illegal characters in hostname
 FAIL a.href should be https://www.my"d(){}|~om?ain#com/path/testurl.html#middle. Was https://www.my%22d%28%29%7B%7D%7C%7Eom/?ain#hash#value.
 Set hash to null
-FAIL a.href should be https://www.mydomain.com/path/testurl.html#. Was https://www.mydomain.com/path/testurl.html.
+PASS a.href is 'https://www.mydomain.com/path/testurl.html#null'
 Set hash to empty string
 PASS a.href is 'https://www.mydomain.com/path/testurl.html#'
 Add hash to mailto: protocol
 FAIL a.href should be mailto:e-mail_address@goes_here#hash-value. Was mailto:e-mail_address@goes_here.
 Add hash to file: protocol
 PASS a.href is 'file:///some%20path#hash value'
 Set hash to '#'
 PASS a.href is 'http://mydomain.com/#'
 Add hash to non-standard protocol
 FAIL a.href should be foo:bar#hash. Was foo:bar.
 PASS successfullyParsed is true
 
 TEST COMPLETE