change use_struct_references behaviour when it would generate invalid type

A possible fix for https://github.com/Khan/genqlient/issues/342. More description and discussion in the issue itself. I am definitely fine for going another approach (reason for draft) if the breaking change is not appropriate.

This PR fixes the issue by changing what type is generated for non-nullable input field types with no default value: for example the added test currently generates:

type UseStructReferencesInput struct {
    Struct         StructInput  `json:"struct"`
    NullableStruct *StructInput `json:"nullableStruct,omitempty"`
}

Previously, it would generate:

type UseStructReferencesInput struct {
    Struct         *StructInput `json:"struct,omitempty"`
    NullableStruct *StructInput `json:"nullableStruct,omitempty"`
}

and after https://github.com/Khan/genqlient/pull/338 this would cause an error during generation as the validation is trying to prevent a type that could cause runtime error by omitting a value for a type for which omitting is not possible.

I have:

[x] Written a clear PR title and description (above)
[x] Signed the Khan Academy CLA
[x] Added tests covering my changes, if applicable
[x] Included a link to the issue fixed, if applicable
[ ] Included documentation, for new features
[x] Added an entry to the changelog

Khan / genqlient

change use_struct_references behaviour when it would generate invalid type #343