Refine Nullness for Kotlin functions returning Unit

Should have unspecified nullness like for Java void/Void.

Closes gh-35420

spring-core/src/main/java/org/springframework/core/Nullness.java

@@ -181,10 +181,10 @@ public enum Nullness {
 
 		public static Nullness forMethodReturnType(Method method) {
 			KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
-			if (function != null && function.getReturnType().isMarkedNullable()) {
-				return Nullness.NULLABLE;
+			if (function != null && ReflectJvmMapping.getJavaType(function.getReturnType()) != void.class) {
+				return (function.getReturnType().isMarkedNullable() ? Nullness.NULLABLE : Nullness.NON_NULL);
 			}
-			return Nullness.NON_NULL;
+			return Nullness.UNSPECIFIED;
 		}
 
 		public static Nullness forParameter(Executable executable, int parameterIndex) {

spring-core/src/test/java/org/springframework/core/NullnessTests.java

@@ -377,6 +377,13 @@ public class NullnessTests {
 		Assertions.assertThat(nullness).isEqualTo(Nullness.NULLABLE);
 	}
 
+	@Test
+	void voidClassMethod() throws NoSuchMethodException {
+		var method = JSpecifyProcessor.class.getMethod("voidClassProcess");
+		var nullness = Nullness.forMethodReturnType(method);
+		Assertions.assertThat(nullness).isEqualTo(Nullness.UNSPECIFIED);
+	}
+
 	// Primitive types
 
 	@Test

spring-core/src/test/kotlin/org/springframework/core/NullnessKotlinTests.kt

@@ -37,6 +37,13 @@ class NullnessKotlinTests {
 		Assertions.assertThat(nullness).isEqualTo(Nullness.NULLABLE)
 	}
 
+	@Test
+	fun unitReturnType() {
+		val method = ::unit.javaMethod!!
+		val nullness = Nullness.forMethodReturnType(method)
+		Assertions.assertThat(nullness).isEqualTo(Nullness.UNSPECIFIED)
+	}
+
 	@Test
 	fun nullableParameter() {
 		val method = ::nullable.javaMethod!!
@@ -78,4 +85,7 @@ class NullnessKotlinTests {
 	@Suppress("unused_parameter")
 	fun nonNull(nonNull: String): String = "foo"
 
+	fun unit() {
+	}
+
 }

spring-core/src/testFixtures/java/org/springframework/core/testfixture/nullness/JSpecifyProcessor.java

@@ -37,5 +37,9 @@ public interface JSpecifyProcessor {
 	@NullMarked
 	@NonNull String nonNullMarkedProcess();
 
+	@NullMarked
 	void voidProcess();
+
+	@NullMarked
+	void voidClassProcess();
 }