NbSelect issue when using it with a template variable

### Issue type

**I'm submitting a:

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
When using the select component together with a template variable is causes and infinite loop on change detection if we try to access the `selected` value

**Expected behavior:**
The component should work as expected and return undefined or null if no value is selected

**Steps to reproduce:**
```

First
Second

First Value Selected

```

**Related code:**
After looking at the source code I found out that select component tries do access selectionModel array while it is still undefined due to data loading.
As a workarround we can do something like:
`

First Value Selected

`

### Other information:

**npm, node, OS, Browser**
```
Node: v18.16.0
npm: 9.5.1
```

**Angular, Nebular**
```
nebular: 11.0.0
angular: 15.2.8
```