Include an optional null variant in the example

Signed-off-by: Michał Górny <mgorny@quansight.com>

Author: mgorny

peps/pep-0825.rst

@@ -339,6 +339,9 @@ like:
       "variants": {
         // REQUIRED: entries for all wheel variants for the package version
 
+        // if a null variant is present
+        "null": {},
+
         // "x8664v3_openblas" label corresponds to:
         // - blas_lapack :: library :: openblas
         // - x86_64 :: level :: v3