Empty media entities with simple widget in paragraphs

Concerns

1. No test. A regression test in tests/src/FunctionalJavascript/SimpleWidgetTest.php exercising an AJAX round-trip (e.g., a field sibling triggering AJAX without the main submit) and verifying the referenced entity isn't corrupted would be worth having. The Complex widget has this guard for a reason; the same failure mode should be covered for Simple.

2. Quiet regression risk. By parity with Complex this is almost certainly safe, but worth confirming that no current IEF flow relies on Simple's extractFormValues populating widget state during AJAX rebuilds. The Complex widget's long-standing guard suggests the answer is no.

Recommendation

The fix is correct and minimal — it just makes Simple match Complex's well-established behavior. But before merging:
1. Add a regression test.
2. Optional: one-line comment noting the guard mirrors InlineEntityFormComplex.