Remove `Assert#notNull` for nullable Link

Signed-off-by: Tran Ngoc Nhan <ngocnhan.tran1996@gmail.com>

src/main/java/org/springframework/data/web/SlicedResourcesAssembler.java

@@ -143,10 +143,11 @@ public class SlicedResourcesAssembler<T>
 	 * @param slice must not be {@literal null}.
 	 * @param assembler must not be {@literal null}.
 	 * @param link must not be {@literal null}.
-	 * @return
+	 * @return a {@link SlicedModel} must not be {@literal null}.
 	 */
 	public <R extends RepresentationModel<?>> SlicedModel<R> toModel(Slice<T> slice,
 			RepresentationModelAssembler<T, R> assembler, Link link) {
+		Assert.notNull(link, "Link must not be null");
 		return createModel(slice, assembler, link);
 	}
 
@@ -166,15 +167,14 @@ public class SlicedResourcesAssembler<T>
 	 *
 	 * @param slice must not be {@literal null}, content must be empty.
 	 * @param type must not be {@literal null}.
-	 * @param link must not be {@literal null}.
-	 * @return
+	 * @param link can be {@literal null}.
+	 * @return a {@link SlicedModel} must not be {@literal null}.
 	 */
 	public SlicedModel<?> toEmptyModel(Slice<?> slice, Class<?> type, @Nullable Link link) {
 
 		Assert.notNull(slice, "Slice must not be null");
 		Assert.isTrue(!slice.hasContent(), "Slice must not have any content");
 		Assert.notNull(type, "Type must not be null");
-		Assert.notNull(link, "Link must not be null");
 
 		SliceMetadata metadata = asSliceMetadata(slice);
 
@@ -187,6 +187,7 @@ public class SlicedResourcesAssembler<T>
 
 	@Deprecated
 	public SlicedModel<?> toEmptyModel(Slice<?> slice, Class<?> type, Optional<Link> link) {
+		Assert.notNull(link, "Link must not be null");
 		return toEmptyModel(slice, type, link.orElse(null));
 	}
 

src/test/java/org/springframework/data/web/SlicedResourcesAssemblerUnitTest.java

@@ -23,6 +23,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 
+import java.util.Optional;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.springframework.data.domain.AbstractPageRequest;
@@ -259,6 +260,32 @@ class SlicedResourcesAssemblerUnitTest {
 				.isEqualTo("http://localhost/sample?foo=bar&page=0&size=1");
 	}
 
+	@Test // GH-3452
+	void emptySliceCreatorNullLink() {
+
+		var result = assembler.toEmptyModel(EMPTY_SLICE, Person.class, (Link) null);
+
+		var content = result.getContent();
+		assertThat(content).hasSize(1);
+
+		var element = content.iterator().next();
+		assertThat(element).isInstanceOf(EmbeddedWrapper.class);
+		assertThat(((EmbeddedWrapper) element).getRelTargetType()).isEqualTo(Person.class);
+	}
+
+	@Test // GH-3452
+	@SuppressWarnings("deprecation")
+	void emptySliceCreatorRejectsNullOptionalLink() {
+
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> assembler.toEmptyModel(EMPTY_SLICE, Person.class, (Optional<Link>) null));
+	}
+
+	@Test // GH-3452
+	void createsSliceRejectsNullLink() {
+		assertThatIllegalArgumentException().isThrownBy(() -> assembler.toModel(createSlice(1), (Link) null));
+	}
+
 	private static Slice<Person> createSlice(int index) {
 
 		Pageable request = PageRequest.of(index, 1);