describe() on schema with default value null results in "Cannot read property 'Symbol(literal)' of null"

[everhardt]

Support plan

• which support plan is this issue covered by? (e.g. Community, Core, Plus, or Enterprise): Community
• is this issue currently blocking your project? (yes/no): yes
• is this issue affecting a production system? (yes/no): no

Context

• node version: v12.12.0
• module version with issue: 16.1.7
• last module version without issue: 14.3.1 (not checked versions in between)
• environment (e.g. node, browser, native): node
• used with (e.g. hapi application, another framework, standalone, ...): standalone
• any other relevant information:

What are you trying to achieve or the steps to reproduce?

I try to get the describe() output of a string schema that has null as its allowed default.

const Joi = require('@hapi/joi');

const schema = Joi.string().allow(null).default(null);

console.log(schema.describe());

What was the result you got?

TypeError: Cannot read property 'Symbol(literal)' of null

What result did you expect?

A nice describe output