Hide 'Add Existing Item' option if there are no valid items to add

## 🎉 Description

* [ ] 🐛 This is a bug report.
* [x] 📈 This is a feature request.

## 📝 Details

We may hide the Add existing item option if there are no items in the collection, same with the items tab.


## 🔢 Steps To Reproduce Issue [ Good To Have ]

```
- Go to the Collections tab
- Open a collection
- Try to add an existing item
```

If it opens a new modal with the message "No valid items found", should we hide the option in the dropdown in the first place?

Start in the Collections tab by reproducing the flow of opening a collection and choosing Add existing item. Trace the dropdown and the "No valid items found" modal; done means the option is hidden when no valid items can be added and remains available when valid items exist.

Written by the indexing model from the issue text.