Fix optional field

[debiff]

This PR allow to identify anonymously a user using this possible permutations:

• name=undefined, email=undefined
• name=defined, email=undefined
• name=undefined, email=defined
• name=defined, email=defined

[Undermaken]

To avoid type redefinition could UserInfo be an extension of AnonymousIdentity?

interface UserInfo extends AnonymousIdentity{
    // user's phone
    phone?: number
    // department to redirect the chat
    department?: string
    // tags for chat
    tags?: Array<string>
  }

[debiff]

Yes, it could. Thank you. Refactored in 4b9b6ef