Fix accidental @Nullable declaration on addAttribute(Object)

Issue: SPR-16831

spring-context/src/main/java/org/springframework/ui/ConcurrentModel.java

@@ -85,10 +85,9 @@ public class ConcurrentModel extends ConcurrentHashMap<String, Object> implement
 	 * the model when using this method because we cannot correctly determine
 	 * the true convention name. View code should check for {@code null} rather
 	 * than for empty collections as is already done by JSTL tags.</i>
-	 * @param attributeValue the model attribute value (never {@code null} for {@code ConcurrentModel},
-	 * with the {@code Nullable} declaration inherited from {@link Model#addAttribute(String, Object)})
+	 * @param attributeValue the model attribute value (never {@code null})
 	 */
-	public ConcurrentModel addAttribute(@Nullable Object attributeValue) {
+	public ConcurrentModel addAttribute(Object attributeValue) {
 		Assert.notNull(attributeValue, "Model attribute value must not be null");
 		if (attributeValue instanceof Collection && ((Collection<?>) attributeValue).isEmpty()) {
 			return this;