SQLite cannot update a column to NULL

Gist of the fix:

diff --git a/includes/database/sqlite/query.inc b/includes/database/sqlite/query.inc
index d456afa..a176ed6 100644
--- a/includes/database/sqlite/query.inc
+++ b/includes/database/sqlite/query.inc
@@ -101,7 +101,7 @@ class UpdateQuery_sqlite extends UpdateQuery {
       }
       elseif (!isset($data)) {
         // The field will be set to NULL.
-        $condition->isNull($field);
+        $condition->isNotNull($field);
       }
       else {
         $condition->condition($field, $data, '<>');

We need to increase the test coverage of UpdateQuery.

Patches attached.