Fixes "replace selection" behavior when nothing is selected (#251)

* Check that selection has text before replacing

* Disable replaceSelection if nothing is selected

* Reverts changes to chat.tsx, modifies selection-watcher per @dlqqq

* Update chat.tsx

Removes unneeded comment

packages/jupyter-ai/src/selection-watcher.ts

@@ -58,6 +58,11 @@ function getTextSelection(widget: Widget | null): Selection | null {
     .getSource()
     .substring(startOffset, endOffset);
 
+  // Do not return a Selection object if no text is selected
+  if (!text) {
+    return null;
+  }
+
   // ensure start <= end
   // required for editor.model.sharedModel.updateSource()
   if (startOffset > endOffset) {