drag test helper division by 0

**Describe the bug**
drag test helper can produce a division by 0, which causes an error for `triggerEvent` because the value is `Infinity`

**To Reproduce**
Reproduction unsure, but only seems to happen in headless test mode.

**Expected behavior**
Avoid the division by 0

**Additional context**
The offending line
```
const scale = clientHeight / (rect.bottom - rect.top);
```
https://github.com/adopted-ember-addons/ember-sortable/blob/master/addon/src/test-support/helpers/drag.js#L61