Disconnect the context-menu when empty or null items

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [X] feature request

### Issue description

**Current behavior:**
When our nbContextMenu is empty (or null), we have to handle an ugly ngIf around that element. Potentially having twice the same tags, but just one with the [nbContextMenu] attribute and one without.

**Expected behavior:**
The nbContextMenu could simply disconnect the `dynamicOverlayHandler` when the item list is empty rather than calling a `validateItems`. That way we could support receiving the menuItems later or not having menuItems at all in some cases.

**Steps to reproduce:**
`[nbContextMenu]="null"` or `[nbContextMenu]="[]"` should just be handled. Nothing should show.

**Related code:**

### Other information:
**Angular, Nebular**
Angular: 8.2.14
Nebular: 4.6.0