Select collapses when size is set but no value is selected

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
When setting a size for a select with no value selected
```html

```
the select component collapses to height=0. This does not happen when not setting the [size] property

**Expected behavior:**
When setting the size and no value is selected, the select should not collapse

**Steps to reproduce:**
This will display the placeholder
```html

```
This collapses down
```html

```
**Related code:**
```
insert short code snippets here
```

### Other information:
node: v12.16.1
npm: 6.13.4
ngx-admin 4.0.1
angular: 8
nebular 4.6.0
Windows 10, Chrome