When we get bored with or irritated by the process of writing we may fall into a no man's land of mental lingering. Here, we say that we want to write, but we cannot think of a single thing. We have established that writing is always hard work, no matter how accomplished we are with the pen or the keyboard, but even so, to say that nothing at all comes to mind seems to me the cry of anxiety and not really a reflection of how switched on our minds are when we sit to work. So it goes that when I came across Cicero I thought that his suggestion would get many writing gears moving, thus proving that there is no such thing as 'nothing to write'.
Pick up the pen and write about nothing.