nb-menu does not expand on initial load

### Issue type

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

* [X] bug report
* [ ] feature request

### Issue description

**Current behavior:**
The nb-menu does not expand properly on initial load. Passing the mouse over it will open it.

**Expected behavior:**
Previous version 4.x, opened the nb-menu properly on initial load.

**Steps to reproduce:**
```
items: NbMenuItem[] = [
{
title: 'Test 1',
link: '/areas',
children: [
{
title: 'Test 1.1',
link: '/areas/login',
}
],
},
{
title: 'Test 2',
link: '/controls',
children: [
{
title: 'Test 2.1',
link: '/controls/login',
}
]
}
];

```
Load the browser on a page like /area/login
It first starts like this:

Then pass your mouse over the nb-menu, and you get this:

### Other information:

**npm, node, OS, Browser**
```
Node: v12.3.1
Npm: 6.9.0
```

**Angular, Nebular**
```
Angular: 9.1.0
CDK: 9.2.0
Nebular: 5.0.0
```