doc: Add example of null to assert.ifError

PR-URL: #18236
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>

Author: Leko

doc/api/assert.md

@@ -337,6 +337,8 @@ argument in callbacks.
 ```js
 const assert = require('assert');
 
+assert.ifError(null);
+// OK
 assert.ifError(0);
 // OK
 assert.ifError(1);