Unconditionally send gtid in SHOW BINARY LOGS WITH GTID

Summary:
When the gtid is empty, nothing is sent over the wire to
the client. This was causing memory issues client side because the
client expects a fixed number of columns per row, and nothing was being sent.
Fix is to send an empty string in it's place.

squash with: 25fec83

Reviewed By: lth

Differential Revision: D4812840

fbshipit-source-id: 103dd4e

sql/rpl_master.cc

@@ -2712,6 +2712,8 @@ bool show_binlogs(THD* thd, bool with_gtid)
         gtid_set.to_string(&buf, &Gtid_set::commented_string_format);
         protocol->store(buf, strlen(buf), &my_charset_bin);
         free(buf);
+      } else {
+        protocol->store("", 0, &my_charset_bin);
       }
     }