Enable TypeScript strictNullChecks (#35843)

A big step towards enabling strict mode in Typescript.

There was definitely a good share of potential bugs while refactoring
this. When in doubt, I opted to keep the potentially broken behaviour.
Notably, the `DOMEvent` type is gone, it was broken and we're better of
with type assertions on `e.target`.

---------

Signed-off-by: silverwind <me@silverwind.io>
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/modules/diff-file.ts

@@ -12,7 +12,7 @@ export type DiffTreeEntry = {
   DiffStatus: DiffStatus,
   EntryMode: string,
   IsViewed: boolean,
-  Children: DiffTreeEntry[],
+  Children: DiffTreeEntry[] | null,
   FileIcon: string,
   ParentEntry?: DiffTreeEntry,
 };
@@ -25,7 +25,7 @@ type DiffFileTree = {
   folderIcon: string;
   folderOpenIcon: string;
   diffFileTree: DiffFileTreeData;
-  fullNameMap?: Record<string, DiffTreeEntry>
+  fullNameMap: Record<string, DiffTreeEntry>
   fileTreeIsVisible: boolean;
   selectedItem: string;
 };