Steps to reproduce the issue

Log in to the administrator interface of a Joomla 3.x install and go to the Users section. Click on the search tools and set -Active- to "Activated"

Expected result

One should see a list of only activated users.

Actual result

Both activated and inactive users are displayed.

System information (as much as possible)

Additional comments

This issue appears to be related to commit c5ede43 which was a previous fix for a slightly different problem. Here the SQL query was modified to include both an empty string and zero. However, the zero here is presented as an integer not a string zero and MySQL is returning rows that clearly have a non empty non-zero field for this query. This is a quick fix the query only needs to be changed to:

$query->where('a.activation IN (' . $db->quote('') . ', ' . $db->quote('0') . ')');

Quoting the zero so that it is treated like a string solves the problem because the activation field is a VARCHAR.