schema BUGFIX handling empty array of features

Beasides NULL, accept also the array of size 1 with the NULL pointer.
diff --git a/src/schema_features.c b/src/schema_features.c
index 3bdd8d4..1e7c7fd 100644
--- a/src/schema_features.c
+++ b/src/schema_features.c
@@ -522,7 +522,7 @@
     uint32_t i = 0;
     struct lysp_feature *f = 0;
 
-    if (!features) {
+    if (!features || !features[0]) {
         /* keep all features disabled */
         return LY_SUCCESS;
     }