Bug 2021: Cannot access S3 bucket root when the access policy checks for empty prefix

https://winscp.net/tracker/2021
(cherry picked from commit a63befb30a719432b1bf48eb39544af934ea330c)

Source commit: dd516f44eb0212e69cd156f7ecb97ad4c123de2e

libs/libs3/src/bucket.c

@@ -712,7 +712,7 @@ void S3_list_bucket(const S3BucketContext *bucketContext, const char *prefix,
 
 
     int amp = 0;
-    if (prefix && *prefix) {
+    if (prefix) {
         safe_append("prefix", prefix);
     }
     if (marker && *marker) {