Description:
------------
This bug is somewhat related to #39213 except it only occurs (as far as I can tell) on fields that are ntext and nvarchar(max). The Moodle bug report MDL-11810 (http://tracker.moodle.org/browse/MDL-11810) has more info. This is with freeTDS.

Reproduce code:
---------------
$tables = array(
    'nulltest' => 'varchar(5)',
    'nulltestnvar' => 'nvarchar(5)',
    'nulltestntext' => 'ntext',
    'nulltestnvarmax' => 'nvarchar(max)'
);
foreach($tables as $table=>$domain) {
    print "==$table==\n";
    mssql_query("create table $table (test $domain);");
    mssql_query("insert into $table values('');");
    $r = mssql_query("select * from $table;");
    while ($a = mssql_fetch_assoc($r)) var_dump($a['test']);
    mssql_query("drop table $table;");
}

Expected result:
----------------
==nulltest==
string(0) ""
==nulltestnvar==
string(0) ""
==nulltestntext==
string(0) ""
==nulltestnvarmax==
string(0) ""

Actual result:
--------------
==nulltest==
string(0) ""
==nulltestnvar==
string(0) ""
==nulltestntext==
NULL
==nulltestnvarmax==
NULL