If I want the sidebar open, I want it open, regardless of whether it might momentarily have no content. Reopening it is an unnecessary extra step.
Automatically closing the sidebar also changes the width of the main editing window, which is disorienting while writing and editing, as the text moves around.
Maybe add a double-tap action to the sidebar icon to pin it open (with visual feedback like a filled sidebar on the icon).
(I am using the iPad app)