Make getters and setters null-safety consistent

This commit ensure that null-safety is consistent between
getters and setters in order to be able to provide beans
with properties with a common type when type safety is
taken in account like with Kotlin.

It also add a few missing property level @Nullable
annotations.

Issue: SPR-15792

spring-jms/src/test/java/org/springframework/jms/listener/AbstractMessageListenerContainerTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.jms.listener;
 
+import org.junit.Assert;
 import org.junit.Test;
 
 /**
@@ -28,10 +29,10 @@ public abstract class AbstractMessageListenerContainerTests {
 
 	protected abstract AbstractMessageListenerContainer getContainer();
 
-
-	@Test(expected = IllegalArgumentException.class)
-	public void testSettingMessageListenerToANullType() throws Exception {
+	
+	public void testSettingMessageListenerToANullType() {
 		getContainer().setMessageListener(null);
+		Assert.assertNull(getContainer().getMessageListener());
 	}
 
 	@Test(expected = IllegalArgumentException.class)