Align execute(TaskCallback) declaration with return value nullability

See gh-36057

spring-core/src/main/java/org/springframework/core/task/SyncTaskExecutor.java

@@ -18,6 +18,8 @@ package org.springframework.core.task;
 
 import java.io.Serializable;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.Assert;
 import org.springframework.util.ConcurrencyThrottleSupport;
 
@@ -70,7 +72,7 @@ public class SyncTaskExecutor extends ConcurrencyThrottleSupport implements Task
 	 * @throws E if propagated from the given {@code TaskCallback}
 	 * @since 7.0
 	 */
-	public <V, E extends Exception> V execute(TaskCallback<V, E> task) throws E {
+	public <V extends @Nullable Object, E extends Exception> V execute(TaskCallback<V, E> task) throws E {
 		Assert.notNull(task, "Task must not be null");
 		if (isThrottleActive()) {
 			beforeAccess();