schema: fix empty compiled shema in iter_children

A module schema can have no compiled schema. In this case, lys_getnext
returns an error (module and parent being NULL), leading to this log
"Invalid argument parent || module || ext (lys_getnext_()).".

Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>

Author: samuel-gauthier

libyang/schema.py

@@ -1647,6 +1647,8 @@ def _skip(node) -> bool:
         return False
 
     if ffi.typeof(parent) == ffi.typeof("struct lys_module *"):
+        if parent.compiled == ffi.NULL:
+            return
         module = parent.compiled
         parent = ffi.NULL
     else: