radicle: Allow to clear issue descriptions and comments

Previously it was possible to create an issue with an empty description
(i.e. an empty "root" comment), and also to change it later to a
non-empty string, however it was not possible to clear it again.

crates/radicle/src/cob/thread.rs

@@ -532,9 +532,6 @@ pub fn edit<L>(
     body: String,
     embeds: Vec<Embed<Uri>>,
 ) -> Result<(), Error> {
-    if body.is_empty() {
-        return Err(Error::Edit(id));
-    }
     debug_assert!(!thread.timeline.contains(&id));
     thread.timeline.push(id);