[Context Menu] Not centered context menu

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**

When creating a `` component only showing the picture (`onlyPicture=true`), the corresponding context menu is not centered under the picture. It looks like, even without showing the title or name of the user, a `div` with the class `info-container` is created which has a `margin-left: .5rem` attached. Without it, it works as expected.

**Expected behavior:**

Context menu is centered under the picture, even without showing the name and title of the user.

**Steps to reproduce:**

In your HTML code insert:
`



`

**Related code:**


### Other information:

Node: v8.11.2
OS: Windows 10 Pro
Browser: Chrome

**Angular, Nebular**

Angular: 6.0.2
nebular/theme: 2.0.0-rc.8