doc: Add example of null to assert.ifError

PR-URL: https://github.com/nodejs/node/pull/18236
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api/assert.md

@@ -442,6 +442,8 @@ argument in callbacks.
 ```js
 const assert = require('assert').strict;
 
+assert.ifError(null);
+// OK
 assert.ifError(0);
 // OK
 assert.ifError(1);