Backport Specification.unrestricted() from main branch.

This would allow 3.5 users to rework their code ahead of the 4.0 release. 'where(null)' was deprecated and should be replaced with unrestricted()

Author: r-gruber

spring-data-jpa/src/main/java/org/springframework/data/jpa/domain/Specification.java

@@ -44,6 +44,16 @@ public interface Specification<T> extends Serializable {
 
 	@Serial long serialVersionUID = 1L;
 
+	/**
+	 * Simple static factory method to create a specification matching all objects.
+	 *
+	 * @param <T> the type of the {@link Root} the resulting {@literal Specification} operates on.
+	 * @return guaranteed to be not {@literal null}.
+	 */
+	static <T> Specification<T> unrestricted() {
+		return (root, query, builder) -> null;
+	}
+
 	/**
 	 * Negates the given {@link Specification}.
 	 *
@@ -72,7 +82,8 @@ static <T> Specification<T> not(@Nullable Specification<T> spec) {
 	 * @param spec can be {@literal null}.
 	 * @return guaranteed to be not {@literal null}.
 	 * @since 2.0
-	 * @deprecated since 3.5, to be removed with 4.0 as we no longer want to support {@literal null} specifications.
+	 * @deprecated since 3.5, to be removed with 4.0 as we no longer want to support {@literal null} specifications. Use
+	 *             {@link #unrestricted()} instead.
 	 */
 	@Deprecated(since = "3.5.0", forRemoval = true)
 	static <T> Specification<T> where(@Nullable Specification<T> spec) {