After upgrading from Spring Boot 3 to 4: Serialized ProblemDetail is missing the 'type' field when it's not specified

[vprudnikov]

Spring Boot 4.1.0

With Spring Boot 3, a ProblemDetail instance with type=null produced JSON with "type": "about:blank".
After upgrading to Spring Boot 4, there is no type field in the serialized JSON.

For example:

{
  "detail" : "Test detail",
  "instance" : "/TestException",
  "status" : 400,
  "title" : "Non-compliant Test Rule"
}

[bclozel]

This is expected, see #35294 (comment)