dict BUGFIX allow storing "" strings

Allow storing 0 length strings in dictionary, since they are valid
values for some cases.
diff --git a/src/dict.c b/src/dict.c
index f9751cd..e4887e4 100644
--- a/src/dict.c
+++ b/src/dict.c
@@ -209,7 +209,7 @@
         len = strlen(value);
     }
 
-    if (!value || !len) {
+    if (!value) {
         return NULL;
     }
     return dict_insert(ctx, (char *)value, len, 0);
@@ -218,10 +218,8 @@
 const char *
 lydict_insert_zc(struct ly_ctx *ctx, char *value)
 {
-    int len;
-
-    if (!value || !(len = strlen(value))) {
+    if (!value) {
         return NULL;
     }
-    return dict_insert(ctx, value, len, 1);
+    return dict_insert(ctx, value, strlen(value), 1);
 }