Default support for "field IS NULL" condition

[luku]

Problem:

echo  get_db()->getTable('Item')->getSelectForFindBy(array('item_type_id' => null));

Expected:

SELECT items.* FROM items WHERE item_type_id IS NULL

Actual:

SELECT items.* FROM items WHERE item_type_id = ?

And then you get error, because null is not passed as extra parameter into prepared statement.

I know, it won't fix calls like:

echo get_db()->getTable('Item')->getSelectForFindBy(array('item_type_id' => array(1, 2, null)));

Should I fix NULL in arrays as well? So it would produce SQL like: WHERE (item_type_id IN (1,2) OR item_type_id IS NULL)?

I'm not 100% sure if this change doesn't break it for other Omeka users calling parent::applySearchFilters() from their plugins, but I checked few plugins (ExhibitBuilder, SimplePages, HistoryLog) and it's OK.