fix(virtualScroll) remove items if array is null

[pablomaurer]

Short description of what this resolves:

If you update the array with arr.length = 0 the items get removed. But when doing it like arr = [] the items will not disappear.

Changes proposed in this pull request:

• When getting _recordSize check if we actually have an array else set to 0
• Run through changes even when there are no records, so the previous items will get removed.
• Be consistent and use everywhere the _recordSize instead _records.length

Ionic Version: 3.x

Fixes: #12034

[pablomaurer]

closed in favor of #12194