Bug fix: increase maximum size of strings concatenated with GROUP_CONCAT()

I noticed an empty value in converted data, which I tracked down to
a limitation in the size of strings concatenated with GROUP_CONCAT().
The default size is 1024, which is reached with 500-1000 authors with
identifiers over 1-4 digits + separator.

Reference:
https://dev.mysql.com/doc/refman/5.0/en/server-system-variables.html#sysvar_group_concat_max_len