disable input validation when use_struct_references is enabled

[Fazt01]

Fix for #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:

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• Added tests covering my changes, if applicable
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[benjaminjkraft]

Great, thanks!