data parser CHANGE make data parser accept empty data

In case a schema doesn't require some mandatory element, having an
empty data tree (represented as NULL) can be completely correct.
diff --git a/src/xml.c b/src/xml.c
index 6f0c222..f466fca 100644
--- a/src/xml.c
+++ b/src/xml.c
@@ -1128,6 +1128,8 @@
     unsigned int len;
     struct lyxml_elem *root, *first = NULL, *next;
 
+    ly_errno = LY_SUCCESS;
+
 #ifndef NDEBUG
     /* TODO: threads support */
     lineno = 1;