Issue #3421296: Guard against undefined array offsets in remove/close flows

Ben Stallingsrequested to merge
issue/inline_entity_form-3421296:3421296-submit-handler-state-purge into 3.x

Two defensive null-coalesce fixes exposed under PHP 8 when rebuild paths hit the IEF close/remove handlers before widget state is fully populated:

  • inline_entity_form_close_all_forms(): tolerate a missing 'entities' substate when closing all forms (can be NULL on fresh rebuilds).
  • submitConfirmRemove(): tolerate a missing 'delete' key on the remove-confirm sub-form values.

Closes #3421296

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