Add support for a nullish test in types

Reviewed-by: Junyoung Choi <fluke8259@gmail.com> 
Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Closes GH-11.

Author: Roang-zero1

index.d.ts

@@ -37,7 +37,12 @@ declare namespace unistUtilIs {
    *
    * @typeParam T type of node that passes test
    */
-  type Test<T extends Node> = TestType<T> | TestObject<T> | TestFunction<T>
+  type Test<T extends Node> =
+    | TestType<T>
+    | TestObject<T>
+    | TestFunction<T>
+    | null
+    | undefined
 }
 
 /**
@@ -57,7 +62,7 @@ declare namespace unistUtilIs {
  */
 declare function unistUtilIs<T extends Node>(
   node: unknown,
-  test: unistUtilIs.Test<T> | Array<unistUtilIs.Test<any>>,
+  test?: unistUtilIs.Test<T> | Array<unistUtilIs.Test<any>>,
   index?: number,
   parent?: Parent,
   context?: any

package.json

@@ -25,7 +25,8 @@
   "author": "Titus Wormer <tituswormer@gmail.com> (https://wooorm.com)",
   "contributors": [
     "Titus Wormer <tituswormer@gmail.com> (https://wooorm.com)",
-    "Christian Murphy <christian.murphy.42@gmail.com>"
+    "Christian Murphy <christian.murphy.42@gmail.com>",
+    "Lucas Brandstaetter <lucas@brandstaetter.tech> (https://github.com/Roang-zero1)"
   ],
   "files": [
     "index.js",

unist-util-is-test.ts

@@ -1,5 +1,7 @@
 import {Node, Parent} from 'unist'
+
 import {Heading} from 'mdast'
+
 import unified = require('unified')
 import is = require('unist-util-is')
 import convert = require('unist-util-is/convert')
@@ -47,8 +49,6 @@ const maybeElement: Element = element
 is()
 // $ExpectError
 is<Node>()
-// $ExpectError
-is<Node>(heading)
 
 /* Incorrect generic. */
 // $ExpectError
@@ -61,6 +61,11 @@ is<Record<string, unknown>>(heading, 'heading')
 /* Should be assignable to boolean. */
 const wasItAHeading: boolean = is<Heading>(heading, 'heading')
 
+/* Test is optional */
+is<Node>(heading)
+is<Node>(heading, null)
+is<Node>(heading, undefined)
+
 /* Should support string tests. */
 is<Heading>(heading, 'heading')
 is<Heading>(element, 'heading')
@@ -190,3 +195,5 @@ convert<Element>({type: 'heading', depth: 2})
 convert<Heading>(isHeading)
 // $ExpectError
 convert<Element>(isHeading)
+convert<Node>(null)
+convert<Node>(undefined)