change use_struct_references behaviour when it would generate invalid type

[Fazt01]

A possible fix for #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 #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:

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