Remove misleading `@Nullable` annotation from `AggregateReference.getId()`.

Aggregate references must be valid on their own and a null reference would represent a broken reference as we consider null identifiers as transient.

Closes #2104

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/mapping/AggregateReference.java

@@ -31,14 +31,21 @@ import org.springframework.util.Assert;
  */
 public interface AggregateReference<T, ID> {
 
+	/**
+	 * Creates an {@link AggregateReference} that refers to the target aggregate root with the given id.
+	 *
+	 * @param id aggregate identifier. Must not be {@literal null}, can be a simple value.
+	 * @return
+	 * @param <T> target aggregate type.
+	 * @param <ID> target aggregate identifier type.
+	 */
 	static <T, ID> AggregateReference<T, ID> to(ID id) {
 		return new IdOnlyAggregateReference<>(id);
 	}
 
 	/**
-	 * @return the id of the referenced aggregate. May be {@code null}.
+	 * @return the id of the referenced aggregate.
 	 */
-	@Nullable
 	ID getId();
 
 	/**
@@ -55,7 +62,6 @@ public interface AggregateReference<T, ID> {
 		public IdOnlyAggregateReference(ID id) {
 
 			Assert.notNull(id, "Id must not be null");
-
 			this.id = id;
 		}
 
@@ -82,7 +88,6 @@ public interface AggregateReference<T, ID> {
 
 		@Override
 		public String toString() {
-
 			return "IdOnlyAggregateReference{" + "id=" + id + '}';
 		}
 	}