Nullability fine-tuning around bean properties

Issue: SPR-15720
Issue: SPR-15792

spring-core/src/main/java/org/springframework/util/CustomizableThreadCreator.java

@@ -41,6 +41,7 @@ public class CustomizableThreadCreator implements Serializable {
 
 	private boolean daemon = false;
 
+	@Nullable
 	private ThreadGroup threadGroup;
 
 	private final AtomicInteger threadCount = new AtomicInteger(0);

spring-core/src/main/java/org/springframework/util/concurrent/ListenableFutureCallbackRegistry.java

@@ -132,7 +132,7 @@ public class ListenableFutureCallbackRegistry<T> {
 	 * added callbacks with the given result.
 	 * @param result the result to trigger the callbacks with
 	 */
-	public void success(T result) {
+	public void success(@Nullable T result) {
 		synchronized (this.mutex) {
 			this.state = State.SUCCESS;
 			this.result = result;

spring-core/src/main/java/org/springframework/util/xml/AbstractXMLReader.java

@@ -144,6 +144,7 @@ abstract class AbstractXMLReader implements XMLReader {
 	 * handler. The property name for a lexical handler is {@code http://xml.org/sax/properties/lexical-handler}.
 	 */
 	@Override
+	@Nullable
 	public Object getProperty(String name) throws SAXNotRecognizedException, SAXNotSupportedException {
 		if ("http://xml.org/sax/properties/lexical-handler".equals(name)) {
 			return this.lexicalHandler;