NULL_BOOLEAN is not compatible with Value Objects

[cushon]

google-http-client uses sentinel objects like NULL_BOOLEAN to distinguish between fields being absent and explicitly set to null:

https://github.com/googleapis/google-http-java-client/blame/95fac312dd743a94fb28532ef8a50d374e6481ad/google-http-client/src/main/java/com/google/api/client/util/Data.java#L48-L49

Those sentinel fields are using deprecated constructors like new Boolean(true) (the deprecation warnings were suppressed in #1215).

JDK 28 introduces a preview of the value classes feature (https://openjdk.org/jeps/401). With JEP 401 boxed primitive classes like Boolean and Integer become value classes that don't have identity, so instance are compared by value:

• new Boolean(true) == Boolean.TRUE will evaluate to true
• new Integer(0) == Integer.valueOf(0) will evaluate to true

This causes google-http-client's Data.isNull(object) to return true for legitimate values.

With JEP 401, assertions like these will start to fail:

    assertThat(Data.isNull(true)).isFalse(); 
    assertThat(Data.isNull(0)).isFalse();
    assertThat(Data.isNull(0L)).isFalse();

Possible fixes:

• Introduce a new API like JsonNullable<Boolean> to replace the use of NULL_BOOLEAN
• Maintain a set of explicitly set fields, that could be queried instead of relying on sentinel values.