Add an 'empty' RecipeChoice for certain recipes and ingredient slots

[Machine-Maker]

Adds RecipeChoice#empty which is allowed in specific recipes and ingredient slots.

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Download the paperclip jar for this pull request: paper-10710.zip

[tal5]

There's still an issue with the Preconditions.checkArgument(!result.isEmpty(), "Recipe cannot have an empty result.") check in the SmithingRecipe constructor failing for some vanilla recipes, which can be replicated by just going over them, e.g. getServer().recipeIterator().forEachRemaining(recipe -> getLogger().info("")).
Not super familiar with the recipe system, but from what I can see only SmithingTransformRecipes actually have a result, and SmithingTrimRecipes just pass in an air ItemStack which triggers that check (I.e. can probably just move the check into SmithingTransformRecipe?)

[Machine-Maker]

There's still an issue with the Preconditions.checkArgument(!result.isEmpty(), "Recipe cannot have an empty result.")

Yeah, fixed that directly on master.

It did make me think about another possible error, and it turns out that upstream was just gonna insert null into RecipeChoice parameters in constructors if the nms Ingredient was empty. And that was gonna error. Now no vanilla recipes do that, but recipes added via datapacks surely could. So I've updated this PR to correctly handle converting an nms Ingredient back into an empty RecipeChoice. I also added a test that just runs the conversion for all recipes on the server which should catch changes like this in the future.