Constistent unrestricted() behaviour for all *Specification types.

Closes #4203
Original pull request: #4024

Signed-off-by: Peter Aisher <peter.aisher@gmail.com>

Author: peteraisher

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

@@ -29,6 +29,13 @@
 
 /**
  * Specification in the sense of Domain Driven Design.
+ * <p>
+ * Specifications can be composed into higher order functions from other specifications using
+ * {@link #and(Specification)}, {@link #or(Specification)} or factory methods such as {@link #allOf(Iterable)}.
+ * <p>
+ * Composition considers whether one or more specifications contribute to the overall predicate by returning a
+ * {@link Predicate} or {@literal null}. Specifications returning {@literal null}, such as {@code where(null)}, are
+ * considered to not contribute to the overall predicate, and their result is not considered in the final predicate.
  *
  * @author Oliver Gierke
  * @author Thomas Darimont
@@ -38,6 +45,7 @@
  * @author Jens Schauder
  * @author Daniel Shuy
  * @author Sergey Rukin
+ * @author Peter Aisher
  */
 @FunctionalInterface
 public interface Specification<T> extends Serializable {
@@ -59,7 +67,7 @@ static <T> Specification<T> not(@Nullable Specification<T> spec) {
 				: (root, query, builder) -> {
 
 			Predicate predicate = spec.toPredicate(root, query, builder);
-					return predicate != null ? builder.not(predicate) : builder.disjunction();
+					return predicate != null ? builder.not(predicate) : null;
 		};
 	}
 

spring-data-jpa/src/test/java/org/springframework/data/jpa/domain/SpecificationUnitTests.java

@@ -45,6 +45,7 @@
  * @author Jens Schauder
  * @author Mark Paluch
  * @author Daniel Shuy
+ * @author Peter Aisher
  */
 @SuppressWarnings("removal")
 @ExtendWith(MockitoExtension.class)
@@ -213,15 +214,13 @@ void orCombinesSpecificationsInOrder() {
 		verify(builder).or(firstPredicate, secondPredicate);
 	}
 
-	@Test // GH-3849
+	@Test // GH-3849, GH-4023
 	void notWithNullPredicate() {
 
-		when(builder.disjunction()).thenReturn(mock(Predicate.class));
+		Specification<Object> notSpec = Specification.not(Specification.unrestricted());
 
-		Specification<Object> notSpec = Specification.not((r, q, cb) -> null);
-
-		assertThat(notSpec.toPredicate(root, query, builder)).isNotNull();
-		verify(builder).disjunction();
+		assertThat(notSpec.toPredicate(root, query, builder)).isNull();
+		verifyNoInteractions(builder);
 	}
 
 	static class SerializableSpecification implements Serializable, Specification<Object> {