WoLpH's answer is correct. My own question was somehow "incorrect"... :-)
In fact, I am dealing with a treegrid (Ext.ux.tree.TreeGrid), not a common ExtJS grid (Ext.grid.GridPanel).
The columns of the treegrid are not of Ext.grid.Column type, as the common grid. They are of Ext.list.Column type.
This means that the renderer configuration option does not apply (it exists and works on Ext.grid.Column, but not on Ext.list.Column).
To change the rendering of a Ext.list.Column, there is the tpl configuration option available, which uses the Ext.XTemplate mechanism.
Below, I share the configuration of my treegrid column that did what I wanted:
tpl: '<tpl if="url === null"></tpl><tpl if="url !== null">{url}</tpl>'
The configuration above made my column render an empty string for NULL values, instead of a "null" string, in the treegrid!
Final note: I voted on WoLpH's answer because it is correct. My question was misleading, and I preferred not to change/edit it, but to share my own findings as another answer. I also thank him because it made me focus on the right things. Thanks.