fix: avoid asserting on exact page token value

The page token value changes as of Go 1.21, likely due to changes in Gob
encoding. Old values should still be unmarshaled properly by Go 1.21, so
shouldn't be an issue.

Avoid asserting on the exact page token value in a test, and instead log
if it is empty or not, as the test will produce different outputs
depending on the Go version used.

examples/examplelibrary/listshelves_test.go

@@ -27,7 +27,7 @@ func ExampleServer_ListShelves() {
 	for _, shelf := range page1.Shelves {
 		fmt.Println(shelf.Name, shelf.Theme)
 	}
-	fmt.Println(page1.NextPageToken)
+	fmt.Println("page1.NextPageToken non-empty:", page1.NextPageToken != "")
 	page2, err := server.ListShelves(ctx, &library.ListShelvesRequest{
 		PageSize:  2,
 		PageToken: page1.NextPageToken,
@@ -38,10 +38,11 @@ func ExampleServer_ListShelves() {
 	for _, shelf := range page2.Shelves {
 		fmt.Println(shelf.Name, shelf.Theme)
 	}
-	fmt.Println(page2.NextPageToken)
+	fmt.Println("page2.NextPageToken non-empty:", page2.NextPageToken != "")
 	// Output:
 	// shelves/0001 Sci-Fi
 	// shelves/0002 Horror
-	// Nv-BAwEBCVBhZ2VUb2tlbgH_ggABAgEGT2Zmc2V0AQQAAQ9SZXF1ZXN0Q2hlY2tzdW0BBgAAAAv_ggEEAfyaywRCAA==
+	// page1.NextPageToken non-empty: true
 	// shelves/0003 Romance
+	// page2.NextPageToken non-empty: false
 }