Add testcase from is-my-json-valid#161

Refs: mafintosh/is-my-json-valid#161
Co-authored-by: Gleb Bahmutov <gleb.bahmutov@gmail.com>

test/misc.js

@@ -244,6 +244,28 @@ tape('custom format', function(t) {
   t.end()
 })
 
+tape('custom format string or null', function(t) {
+  const validate = validator(
+    {
+      type: 'object',
+      properties: {
+        foo: {
+          type: ['string', 'null'],
+          format: 'as',
+        },
+      },
+    },
+    { formats: { as: /^a+$/ } }
+  )
+
+  t.notOk(validate({ foo: '' }), 'not as')
+  t.notOk(validate({ foo: 'bar' }), 'not as')
+  t.notOk(validate({ foo: 123 }), 'not as if number')
+  t.ok(validate({ foo: 'a' }), 'as')
+  t.ok(validate({ foo: null }), 'as')
+  t.end()
+})
+
 tape('custom format function', function(t) {
   const validate = validator(
     {