After upgrading to DNN 4.9.0 the FCK editor saved the following code:
<p> </p>
when you did not enter any text.
Because some of my modules use the fact that there is no text in certain fields for showing or not showing some content, I wanted the FCK editor to save nothing ("") instead of the mentioned code.
After some research I found out you can do this by editing the fckconfig.js file, and set the FCKConfig.FillEmptyBlocks option to false.
You can find fckconfig.js in the "/providers/htmleditorproviders/fck/custom/" folder