nbInput missing outline on :focus

### Issue type

**I'm submitting a**

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
When focusing on an nbInput there is no outline.
The same is happening in the [examples](https://akveo.github.io/nebular/docs/components/input/examples#nbinputdirective).

**Expected behavior:**
When focusing on an nbInput there should have an outline.
It is working properly in [ngx-admin](https://www.akveo.com/ngx-admin/pages/forms/inputs).

**Related code:**
```

```

### Other information:

**npm, node, OS, Browser**
```
Node, npm: `v10.16.2` and `6.13.7`
OS: macOS (Catalina 10.15.3)
Browser: all
```

**Angular, Nebular**
```
Angular 9
Nebular v4.6.0
```