Resolve the warnings from TS diagnostics during the tests

Currently, it doesn't make the tests fail because of `warnOnly` flag in the `jest.config.js`.

```
"ts-jest": {
diagnostics: {
warnOnly: true
}
}
```

Warning example

```
ts-jest[ts-compiler] (WARN) TypeScript diagnostics (customize using `[jest-config].globals.ts-jest.diagnostics` option):
src/key/__test__/MemoryKeyStore.spec.ts:60:46 - error TS2345: Argument of type 'string | null' is not assignable to parameter of type 'string'.
```