builtin-string.go: removed useless err == nil check

builtin-string.go

@@ -25,7 +25,7 @@ func doJoinStr(funcName string, sep string, it Iterator) (result any, err error)
 			return
 		}
 	}
-	if err == nil || err == io.EOF {
+	if err == io.EOF {
 		err = nil
 		result = sb.String()
 	}