Syntax to include null or blank within "IN" list?

What is the proper syntax to include a blank or null option within an IN list?
I've unsuccessfully tried various options with the intent to say:: If CLASS-A = *ABC *and CLASS-B is either VALUE1, VALUE2, *VALUE3 *-- or there is NO CLASS-B...

I've tried NULL, null, Null, as well as "" and " " - thought maybe there is something obvious I am missing.

calcrt.get("CLASS-A") == ("ABC") &&
    calcrt.get("CLASS-B") in ("VALUE1" , "VALUE2" , "VALUE3", NULL)

Appreciate any guidance - total noob here!

You can use both null and "" in a list with in operator:

MyObject( someValue in (null, "", "1", "something", "hello world!") )

Will match:

• MyObject{ someValue: null }
• MyObject{ someValue: "" }
• MyObject{ someValue: "1" }
• MyObject{ someValue: "something" }
• MyObject{ someValue: "hello world!" }

Will not match:

• MyObject{ someValue: " " } (whitespace is not null or an empty string)

That being said, if you're working with a Map and you're checking that something is not present, you may want to consider the case that a 'null' for get("Class-B") might not mean the key is not present, but rather that the value is null. In that case you might consider checking that "Class-B" is present in the keyset.

rule "no class-b"
when
  $inputs: Map( this["CLASS-A"] == "ABC" )
  Set( this not contains "CLASS-B" ) from $inputs.keySet()
then
  System.out.println("CLASS-A is ABC and there is no CLASS-B");
end

rule "class-b meets some criteria"
when
  $inputs: Map( this["CLASS-A"] == "ABC",
                this["CLASS-B"] in ("VALUE1", "VALUE2", "VALUE3") )
then
  System.out.println("CLASS-A is ABC and CLASS-B is VALUE1/2/3");
end