fix: typescript return null

[erichbehrens]

These methods can return null, but the typing is wrong, which prevents typescript/eslint to perform proper validation. For instance, the following code does not pass the @typescript-eslint/no-unnecessary-condition rule.

if (oidcService.getAccessToken() === null) { ... }

Unnecessary conditional, the types have no overlap.ESLint[@typescript-eslint/no-unnecessary-condition]