Fix handling of null predicate in Specification.not().

When toPredicate() returns null, Specification.not() now returns
builder.disjunction() instead of builder.not(null). This change
ensures proper handling of null predicates in negated specifications.

Closes #3849
Original pull request: #3856

Signed-off-by: SWQXDBA <983110853@qq.com>

Author: SWQXDBA

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

@@ -156,7 +156,7 @@ static <T> Specification<T> not(Specification<T> spec) {
 		return (root, query, builder) -> {
 
 			Predicate predicate = spec.toPredicate(root, query, builder);
-			return predicate != null ? builder.not(predicate) : null;
+			return predicate != null ? builder.not(predicate) : builder.disjunction();
 		};
 	}