plugins types FEATURE sort callback in empty

... implemented via new lyplg_type_sort_simple().
diff --git a/src/plugins_types.c b/src/plugins_types.c
index 9ebcf48..8305691 100644
--- a/src/plugins_types.c
+++ b/src/plugins_types.c
@@ -29,6 +29,7 @@
 #include "dict.h"
 #include "ly_common.h"
 #include "path.h"
+#include "plugins_internal.h"
 #include "schema_compile.h"
 #include "set.h"
 #include "tree.h"
@@ -314,6 +315,27 @@
     return LY_ENOT;
 }
 
+LIBYANG_API_DEF int
+lyplg_type_sort_simple(const struct ly_ctx *ctx, const struct lyd_value *val1, const struct lyd_value *val2)
+{
+    int cmp;
+    const char *can1, *can2;
+
+    can1 = lyd_value_get_canonical(ctx, val1);
+    can2 = lyd_value_get_canonical(ctx, val2);
+    if (can1 && can2) {
+        cmp = strcmp(can1, can2);
+        return cmp;
+    } else if (!can1 && can2) {
+        return -1;
+    } else if (can1 && !can2) {
+        return 1;
+    } else {
+        /* !can1 && !can2 */
+        return 0;
+    }
+}
+
 LIBYANG_API_DEF const void *
 lyplg_type_print_simple(const struct ly_ctx *UNUSED(ctx), const struct lyd_value *value, LY_VALUE_FORMAT UNUSED(format),
         void *UNUSED(prefix_data), ly_bool *dynamic, size_t *value_len)
diff --git a/src/plugins_types.h b/src/plugins_types.h
index f056643..a6851e8 100644
--- a/src/plugins_types.h
+++ b/src/plugins_types.h
@@ -658,6 +658,12 @@
         const struct lyd_value *val2);
 
 /**
+ * @brief Implementation of ::lyplg_type_sort_clb for a generic simple type.
+ */
+LIBYANG_API_DEF int lyplg_type_sort_simple(const struct ly_ctx *ctx, const struct lyd_value *val1,
+        const struct lyd_value *val2);
+
+/**
  * @brief Implementation of ::lyplg_type_print_clb for a generic simple type.
  */
 LIBYANG_API_DECL const void *lyplg_type_print_simple(const struct ly_ctx *ctx, const struct lyd_value *value,
diff --git a/src/plugins_types/empty.c b/src/plugins_types/empty.c
index 45062d8..b7b82dc 100644
--- a/src/plugins_types/empty.c
+++ b/src/plugins_types/empty.c
@@ -93,7 +93,7 @@
         .plugin.store = lyplg_type_store_empty,
         .plugin.validate = NULL,
         .plugin.compare = lyplg_type_compare_simple,
-        .plugin.sort = NULL,
+        .plugin.sort = lyplg_type_sort_simple,
         .plugin.print = lyplg_type_print_simple,
         .plugin.duplicate = lyplg_type_dup_simple,
         .plugin.free = lyplg_type_free_simple,