Conform more of the code base to strict null checking (#10147)

* Conform more of the code base to strict null checking

* More strict fixes

* More strict work

* Fix missing optional type

* Iterate

src/hooks/useHover.ts

@@ -17,7 +17,7 @@ limitations under the License.
 import { useState } from "react";
 
 export default function useHover(
-    ignoreHover?: (ev: React.MouseEvent) => boolean,
+    ignoreHover: (ev: React.MouseEvent) => boolean,
 ): [boolean, { onMouseOver: () => void; onMouseLeave: () => void; onMouseMove: (ev: React.MouseEvent) => void }] {
     const [hovered, setHoverState] = useState(false);