Currently `TemplateModelVariantProvider.loadModelVariant` will throw `a ModelProviderException` if it doesn't have a model already in the map - however this stops any other mods from providing custom models that are later in their chain.
While this behaviour isn't documented (fabric's `ModelVariantProvider` interface doesn't even *mention* `ModelProviderException`) it does say that it should `@return The loaded UnbakedModel, or null if this ModelVariantProvider doesn't handle a specific Identifier`
