Resolve the warnings from TS diagnostics during the tests

[joojis]

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'.