Fix nullability declaration of PagedModel.

Fixes GH-3313

Author: odrotbohm

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

@@ -58,7 +58,7 @@ public List<T> getContent() {
 	}
 
 	@JsonProperty("page")
-	public @Nullable PageMetadata getMetadata() {
+	public PageMetadata getMetadata() {
 		return new PageMetadata(page.getSize(), page.getNumber(), page.getTotalElements(),
 				page.getTotalPages());
 	}