Kotlin extractor: use special <nulltype> for null literals

This matches the Java extractor's treatment of these literals, and so enables dataflow type-tracking to avoid special-casing Kotlin. Natively, Kotlin would regard this as kotlin.Nothing?, the type that can only contain null (kotlin.Nothing without a ? can take nothing at all), which gets Java-ified as java.lang.Void, and this will continue to be used when a null type has to be "boxed", as in representing substituted generic constraints with no possible type.

java/kotlin-extractor/src/main/kotlin/KotlinFileExtractor.kt

@@ -5745,7 +5745,14 @@ open class KotlinFileExtractor(
     ) =
         exprIdOrFresh<DbNullliteral>(overrideId).also {
             val type = useType(t)
-            tw.writeExprs_nullliteral(it, type.javaResult.id, parent, idx)
+            // Match Java by using a special <nulltype> for nulls, rather than Kotlin's view of this which is
+            // kotlin.Nothing?, the type that can only contain null.
+            val nullTypeName = "<nulltype>"
+            val javaNullType = tw.getLabelFor(
+                "@\"type;$nullTypeName\"",
+                { tw.writePrimitives(it, nullTypeName) }
+            )
+            tw.writeExprs_nullliteral(it, javaNullType, parent, idx)
             tw.writeExprsKotlinType(it, type.kotlinResult.id)
             extractExprContext(it, locId, callable, enclosingStmt)
         }