Nebular Stepper vertical height

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**

When using a vertical stepper, the line connecting the stepper icons varies in length as I step through all the steps. The height of the line appears to be related to the height of the elements in the content of the step.

**Expected behavior:**

It would be better if the line between the step icons was constant while stepping through all the steps. Or, have an option for this.

**Steps to reproduce:**

Please run the StackBlitz project link below and step through each steps. You see that the spacing varies.

**Related code:**
See file `home.component.html` in this [StackBlitz Project](https://stackblitz.com/edit/nebular-stepper-vertical?file=src%2Fapp%2Fhome%2Fhome.component.html).

### Other information:

npm version: 7.11.2
node version: v14.16.1
OS: Ubuntu 21.04
Browser: Chrome 90.0.4430.93

**npm, node, OS, Browser**

**Angular, Nebular**
@angular/core: 11.2.12
@nebular/theme: 7.0.0