QGIS/resources/function_help/json/NULL

{
  "name": "NULL",
  "type": "value",
  "description": "Equates to a null value.",
  "examples": [
       { "expression":"NULL", "returns":"a null value" }
  ],
  "notes": "To test for NULL use an <pre>IS NULL</pre> or <pre>IS NOT NULL</pre> expression."
}