Add section on empty variants in basic data types tutorial

Author: neuroevolutus

data/tutorials/language/0it_01_basic_datatypes.md

@@ -482,6 +482,16 @@ Note that:
 - `unit` is a variant with a unique constructor, which does not carry data: `()`.
 - `bool` is also a variant with two constructors that doesn't carry data: `true` and `false`.
 
+#### Empty Variants
+
+Variants can optionally be defined with absolutely no constructors at all.
+
+```ocaml
+type void = |
+```
+
+Such types are not ordinarily useful in OCaml programs (as they do not have any constructible values), but they can be useful as temporary placeholders when defining types in a [functor](/docs/functors#writing-your-own-functors).
+
 #### Constructors With Data
 
 It is possible to wrap data in constructors. The following type has several constructors with data (e.g., `Hash of string`) and some without (e.g., `Head`). It represents the different means to refer to a Git [revision](https://git-scm.com/docs/gitrevisions).