hash table CHANGE accept null context

Because lydic_remove in old libyang does.
diff --git a/src/hash_table.c b/src/hash_table.c
index 7c1266a..e1a537e 100644
--- a/src/hash_table.c
+++ b/src/hash_table.c
@@ -165,7 +165,7 @@
     struct dict_rec rec, *match = NULL;
     char *val_p;
 
-    if (!value) {
+    if (!ctx || !value) {
         return LY_SUCCESS;
     }