[name]ModelSelector requires passing 'undefined' instead of empty for non-primitive fields

[wtuminski]

I tried to upgrade mst-gql in my project, but when I did so it turned out that I must pass undefined to every empty selector builder of a non-primitive field. In many queries, I want to get only a reference so when invoking the selector builder for a non-primitive field I pass no arguments. The builder argument is now required, but undefined is acceptable. Can't we return to the optional argument?

[wtuminski]

The solution is introduced in the following PR: #408