Null Children Array in Field Formatter Base

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Previously tagged for tests and issue summary in #4 which still needs to happen for this issue to move forward.

Re-rolled against core 10.2

I believe we shouldn't suppress errors caused by a contrib or custom FieldFormatter. The return type of FieldFormatterInterface::viewElements() is explicitly defined as array, so no formatter should return NULL.

I would close this as "Works as designed."

Agree @vidorado, the proper fix is to find the formatter(s) that return NULL and fix them.