Accidentally emptied a Note, which was then automatically deleted
I'm sorry to say this but I think your note is permanently deleted.
Here's why: 
Deleting text in a note is like deleting text in a Word-document. When you delete all text in a Word-document, you can still undo (using CMD+Z) while the document is open. If you closed the document, the undo history gets deleted. And I thinks iOS Notes works this way. Try to create a new Note, add some text, go back, exit the app (by swiping up) and open it again. Can you undo(shake the device) to remove text? No. Undo history is gone.