Fix crash in repositionTooltips when there are no tooltips

FIX: Fix an issue that caused `repositionTooltips` to crash when it was
called on an editor without tooltips.

See https://discuss.codemirror.net/t/typeerror-cannot-read-properties-of-null-reading-maybemeasure/7094

src/tooltip.ts

@@ -739,5 +739,6 @@ export const closeHoverTooltips = closeHoverTooltipEffect.of(null)
 /// re-positioning or CSS change affecting the editor) that could
 /// invalidate the existing tooltip positions.
 export function repositionTooltips(view: EditorView) {
-  view.plugin(tooltipPlugin)?.maybeMeasure()
+  let plugin = view.plugin(tooltipPlugin)
+  if (plugin) plugin.maybeMeasure()
 }