fix: #3585946 keep "Add another" widget on paragraphs with empty content

When "Hide when not empty" is enabled and an existing paragraph item has empty inner content but is structurally non-empty (its EntityReferenceRevisionsItem has a target_id), clicking "Add another" silently stripped the freshly-added widget. The empty new paragraph matched the trailing-empty trim, even though the user had just explicitly asked for it.

Detect AJAX rebuilds triggered by an "_add_more" button and skip the trim entirely on those rebuilds: when the user explicitly asks for an extra widget, never take it away.

Add a paragraphs-based regression test covering the reported scenario.

Closes #3585946