Have `getContactById` also return `null` since it can return a contact or null

[dprevost-LMI]

getContactById can be a contact or null, so the API definition should be like below

getContactById(contactId: string): Promise<Contact | null>;

It also reflects the documentation about contact being null when it does not exists

getContactById(contactId): Promise - returns contact with defined contactId (or null if it doesn't exist)