Added missing null in the type signature of getIdentityClaims in oauth-service.ts

[md-redwan-hossain]

null is being returned but missing in type signature. This PR solves this issue.

public getIdentityClaims(): Record<string, any> {
  const claims = this._storage.getItem('id_token_claims_obj');
  if (!claims) {
    return null;
  }
  return JSON.parse(claims);
}

[davidmccoy-8451]

Ran into the same issue, this should be included