Skip the PENPOT_SELECTION stamp when the selection is collapsed
or points at blocks that no longer exist: blurring the editor
rebuilds the EditorState with fresh block keys while a delayed
on-change still carries the old keys, which crashed Draft.js
reading getCharacterList of undefined and killed the workspace.
Harden the sibling helpers (removeBlurSelectionEntity,
cursorToEnd, select-all/end builders) the same way and add a
vitest suite to the draft-js package covering stale, empty,
and collapsed selections.
Closes#11667
AI-assisted-by: muse-spark-1.3-contributor
