Fixes "replace selection" behavior when nothing is selected

[JasonWeill]

Fixes #239. To repro:

1. In cell 1, highlight some text, and use the Jupyter AI's chat UI to ask a question, with both the "include selection" and "replace selection" boxes checked.
2. After the prompt runs and the output replaces the selection, run cell 1. Cell 2 is now active, and nothing in cell 2 is selected.
3. Ask a question in the chat UI — note that neither "Include selection" nor "replace selection" appear as options.
4. Verify that the response is not added to either cell 1 or cell 2.

[dlqqq]

There is a more robust solution to this that allows us to fix the bug with minimal changes. The root cause of the issue is that the getTextSelection() function, defined in src/selection-watcher.ts, returns a Selection object even when no text is selected.

In src/selection-watcher.ts, modify the implementation of getTextSelection() to

  ...

  const text = editor.model.sharedModel
    .getSource()
    .substring(startOffset, endOffset);
  
  // add this block
  if (!text) {
    return null;
  }

  ...

and revert the other changes to src/components/chat.tsx. I've tested this locally, though you should verify this change independently as well.

[JasonWeill]

@dlqqq Tested locally, successfully. Thanks for your help!

Changes incorporated per requester