shopDomainUpdate should return errors when empty strings given

[dominik-zeglen]

What I'm trying to achieve

To unset the shop's domain.

Steps to reproduce the problem

1. Execute the following query:

mutation {
  shopDomainUpdate(input: {domain: ""}) {
    errors {
      field
      message
    }
    shop {
      domain {
        host
        url
      }
    }
  }
}

2. Notice that while domain was not updated, API returned no errors.

What I expected to happen

To get an error.

[maarcingebala]

An empty domain would break some functionalities of Saleor such as generating URLs in emails. We rather shouldn't allow setting an empty value there.