schema tree BUGFIX handling empty cases in lys_getnext()

It wasn't checked that case is not empty and its child (NULL) was
returned anyway.
diff --git a/tests/utests/schema/test_schema_common.c b/tests/utests/schema/test_schema_common.c
index 9e70bdc..c959a2e 100644
--- a/tests/utests/schema/test_schema_common.c
+++ b/tests/utests/schema/test_schema_common.c
@@ -46,7 +46,7 @@
                                         "  anyxml seven; action eight {input {leaf eight-input {type string;}} output {leaf eight-output {type string;}}}"
                                         "  notification nine {leaf nine-data {type string;}}}"
                                         "leaf b {type string;} leaf-list c {type string;} list d {config false;}"
-                                        "choice x { leaf e {type string;} case y {leaf f {type string;}}} anyxml g;"
+                                        "choice x { case empty-x; leaf e {type string;} case y {leaf f {type string;}}} anyxml g;"
                                         "rpc h {input {leaf h-input {type string;}} output {leaf h-output {type string;}}}"
                                         "rpc i;"
                                         "notification j {leaf i-data {type string;}}"