MDEV-33283: Binlog Checksum is Zeroed by Zlib if Part of Event Data is Empty

An existing binlog checksum can be overridden to 0 if writing a NULL
payload when using Zlib for the computation. That is, calling into
Zlib's crc32 with empty data initializes an incremental CRC
computation to 0.

This patch changes the Log_event_writer::write_data() to exit
immediately if there is nothing to write, thereby bypassing the
checksum computation. This follows the pattern of
Log_event_writer::encrypt_and_write(), which also exits immediately
if there is no data to write.

Reviewed By:
============
Andrei Elkin <andrei.elkin@mariadb.com>

sql/log_event_server.cc

@@ -916,6 +916,10 @@ int Log_event_writer::write_header(uchar *pos, size_t len)
 int Log_event_writer::write_data(const uchar *pos, size_t len)
 {
   DBUG_ENTER("Log_event_writer::write_data");
+
+  if (!len)
+    DBUG_RETURN(0);
+
   if (checksum_len)
     crc= my_checksum(crc, pos, len);