Strict null check vs/workbench/contrib/tasks

[mjbvz]

Part of #60565

Tracks updating vs/workbench/contrib/tasks to have strict null checking.

Not all of these files are eligible to be included in the strict null build yet due to their dependencies. You do not need to fix errors outside of the contrib/tasks directory

Steps

1. Add ./vs//workbench/contrib/tasks/**/*.ts to the include section of src/tsconfig.strictNullChecks.json
2. Run yarn strict-null-check -- --watch
3. Fix strict null check compiler errors inside of the contrib/tasks directory.
4. Verify normal build still works
5. Revert changes to src/tsconfig.strictNullChecks.json