nb-datepicker next/ previous month navigation jumps

### Issue type

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

* [x ] bug report
* [ ] feature request

### Issue description

**Current behavior:**

Some months view have 6 rows and clicking on next or previous month (icons/ arrows) cause these navigation to move up and down.
[Recorded problem in video](https://youtu.be/2u_MmzAqT2I)

**Expected behavior:**

The next and previous months icons/ arrows (navigation) stick in same place.

**Steps to reproduce:**

Add date picker for an input.

**Related code:**
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