disable input validation when `use_struct_references` is enabled

[Fazt01]

Fix for https://github.com/Khan/genqlient/issues/342. As use_struct_references is incompatible with omitempty and pointer validation, the most simple change is to disable the validation when the use_struct_references is enabled. Note this implementation disables the validation even for types that are not influenced by use_struct_references, but still better than disabling the validation globally.

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

[benjaminjkraft]

Great, thanks!