Perform NullAway build-time checks in spring-web

Also in spring-websocket.

See gh-32475

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

@@ -62,6 +62,7 @@ public abstract class CollectionUtils {
 	 * @param collection the Collection to check
 	 * @return whether the given Collection is empty
 	 */
+	@Contract("null -> true")
 	public static boolean isEmpty(@Nullable Collection<?> collection) {
 		return (collection == null || collection.isEmpty());
 	}

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

@@ -101,6 +101,7 @@ public abstract class ObjectUtils {
 	 * either an Object array or a primitive array.
 	 * @param obj the object to check
 	 */
+	@Contract("null -> false")
 	public static boolean isArray(@Nullable Object obj) {
 		return (obj != null && obj.getClass().isArray());
 	}