Add empty text if children are empty (#3889)

If line 170 is not added issues like https://github.com/ianstormtaylor/slate/issues/3625 start surfacing. 
Because the deserialize example is mostly directly copy-pasted into projects, the added line would save many head-scratching down the lane.

docs/concepts/09-serializing.md

@@ -166,6 +166,10 @@ const deserialize = el => {
   }
 
   const children = Array.from(el.childNodes).map(deserialize)
+  
+  if (children.length === 0) {
+    children = [{ text: '' }]
+  }
 
   switch (el.nodeName) {
     case 'BODY':