Fix empty collection first item index

[zorn-v]

Currently data-num-items does not denotes actually items count, and in fact it is 0 for empty collection AND collection with 1 item.
Because of this first item of empty collection obtain index 1
And if there validation violations on new created collection, indexes are shifted by 1
Because field names looks like Entity[collection][1][field] for very first item, but validation violation is like data.collection[0].field

PS. This PR does not include compiled assets via npm run build

[zorn-v]

Other possible fix is just change in form theme collection_row block without js editing (as I do in my project for now)

'data-num-items': form.children is empty ? 0 : max(form.children|keys),

to

'data-num-items': form.children is empty ? -1 : max(form.children|keys),

but "num items" should contain actual items count I think

[javiereguiluz]

Thanks @zorn-v. While merging, I run make build-assets to rebuild the assets, so everything should be fine.