Comparison method violates its general contract when sorting column has some empty cells

The comparison method does not give a total order when there are empty cells in the sorted column, breaking the sorting.

Steps to reproduce:

• Create a page with the following content: https://up1.xwikisas.com/#wdni8-Iv54wTyAb84U4Zkg
• Click Save and View.
• Open the advanced Sort menu of the table
• Set sorting with `Column A` fisrt, then with `id`. Both in Descending order.

Expected result:

The table is sorted in descending order of values for `Column A`, using `id` as a fallback when the values for `Column A` are equal.

Actual result:

The query fails with a "IllegalArgumentException: Comparison method violates its general contract!" error message.