libyang MAINTENANCE change format of the empty loops

To improve readability and because of processing by the formatting
tools, add explicit empty block to the empty loops (for, while).
diff --git a/src/tree_data_free.c b/src/tree_data_free.c
index 8dda446..bb6231c 100644
--- a/src/tree_data_free.c
+++ b/src/tree_data_free.c
@@ -40,7 +40,7 @@
                 meta->parent->meta = meta->next;
             }
         } else {
-            for (iter = meta->parent->meta; iter->next != meta; iter = iter->next);
+            for (iter = meta->parent->meta; iter->next != meta; iter = iter->next) {}
             if (iter->next) {
                 if (siblings) {
                     iter->next = NULL;
@@ -96,7 +96,7 @@
                 attr->parent->attr = attr->next;
             }
         } else {
-            for (iter = attr->parent->attr; iter->next != attr; iter = iter->next);
+            for (iter = attr->parent->attr; iter->next != attr; iter = iter->next) {}
             if (iter->next) {
                 if (siblings) {
                     iter->next = NULL;
@@ -233,7 +233,7 @@
 
     /* get the first (top-level) sibling */
     if (top) {
-        for (; node->parent; node = (struct lyd_node *)node->parent);
+        for (; node->parent; node = (struct lyd_node *)node->parent) {}
     }
     while (node->prev->next) {
         node = node->prev;