Handle nullable=true with allOf

[pglass]

From #10, support was added handle nullable=true for anyOf and oneOf.

This issue is to add support for allOf and nullable=true:

{
  "allOf": [{ "type": "string" }, { "maxLength": 5 }],
  "nullable": true
}

This could convert to a schema such as the following, or possibly another equivalent:

{
  "oneOf": [
    { "type": "null" },
    { "allOf": [{ "type": "string" }, { "maxLength": 5 }] }
  ]
}