Support widgets with non-numeric form keys

Problem/Motivation

With a limited-cardinality Paragraphs field on a content type, I see this 500 error when visiting the /node/add/<type> form:

InvalidArgumentException: Unable to get a value with a non-numeric delta in a list. in Drupal\Core\TypedData\Plugin\DataType\ItemList->get() (line 101 of core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php).

Steps to reproduce

1. Add a Paragraphs field (or any complex widget that has non-numeric keys) to a content type
2. Visit the "node add" or "node edit" form for that content type
3. See the error

Proposed resolution

Filter non-numeric keys from the list, e.g.:

$deltas = array_filter(Element::children($elements), 'is_numeric');

Remaining tasks

1. Does this deserve test coverage? If so, can you suggest a good test for it?

User interface changes

none

API changes

none

Data model changes

none