Add @Nullable declaration to getDefaultConstructor()

See gh-25367

spring-expression/src/main/java/org/springframework/expression/spel/ast/Indexer.java

@@ -726,6 +726,7 @@ public class Indexer extends SpelNodeImpl {
 			}
 		}
 
+		@Nullable
 		private Constructor<?> getDefaultConstructor(Class<?> type) {
 			try {
 				return ReflectionUtils.accessibleConstructor(type);