Fixed a bug in concat_ws(), which did not add concat separator

in case of an empty string. Bug ID 586.

mysql-test/r/func_str.result

@@ -64,7 +64,7 @@ concat_ws(NULL,'a')	concat_ws(',',NULL,'')
 NULL	
 select concat_ws(',','',NULL,'a');
 concat_ws(',','',NULL,'a')
-a
+,a
 SELECT CONCAT('"',CONCAT_WS('";"',repeat('a',60),repeat('b',60),repeat('c',60),repeat('d',100)), '"');
 CONCAT('"',CONCAT_WS('";"',repeat('a',60),repeat('b',60),repeat('c',60),repeat('d',100)), '"')
 "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";"bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb";"cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc";"dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd"

sql/item_strfunc.cc

@@ -495,18 +495,18 @@ String *Item_func_concat_ws::val_str(String *str)
   str->length(0);				// QQ; Should be removed
   res=str;
 
-  // Skip until non-null and non-empty argument is found.
+  // Skip until non-null argument is found.
   // If not, return the empty string
   for (i=0; i < arg_count; i++)
-    if ((res= args[i]->val_str(str)) && res->length())
+    if ((res= args[i]->val_str(str)))
       break;
   if (i ==  arg_count)
     return &empty_string;
 
   for (i++; i < arg_count ; i++)
   {
-    if (!(res2= args[i]->val_str(use_as_buff)) || !res2->length())
-      continue;					// Skip NULL and empty string
+    if (!(res2= args[i]->val_str(use_as_buff)))
+      continue;					// Skip NULL
 
     if (res->length() + sep_str->length() + res2->length() >
 	current_thd->variables.max_allowed_packet)