Clarify how to handle default value on non-nullable optional properties when "default" keyword is not specified

[ahmednfwela]

Related OpenAPITools/openapi-generator#10972

Spec version: 3.0.3
the spec doesn't indicate how to handle model properties in the following case:

required: false
nullable: false
type: xxxxx

• required: false means that the property might not exist in the json
• nullable: false means that the property doesn't accept null as a value

so what to do when the spec document doesn't specify the default keyword ?

1. assume default is null (works only for properties where nullable: true, otherwise it will break the spec)

2. assume default to be of same type as type, so for non-nullable types that would be
   "boolean" => false
   "object" => { }
   "array" => [ ]
   "number"/"integer" => 0
   "string" => ""

3. make it required to specify the default keyword when required: false and nullable: false

4. ignore the provided required: false and assume instead required: true