Strict null check src/vs/workbench/services/extensions

[mjbvz]

Part of #60565

Tracks updating src/vs/workbench/services/extensions/ 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 extensions directory

Steps

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

[mjbvz]

I took care of this in 7d0e64f

Please take a look at those changes and revise them as needed