Provide better error message if file-key-management-filename is not set

Right now if file-key-management-filename is unset/empty (which is probably the default state if you naively try to install the plugin the first time) installing the file-key-management plugin fails with the unintuitive error message

MariaDB [test]> install soname 'file_key_management';

ERROR 29 (HY000): File '' not found (Errcode: 2 "No such file or directory")

I'd suggest to check the filename variable for being empty before trying to pass it as a filename, and to issue a different error message if unset:

MariaDB [(none)]> install soname 'file_key_management';

ERROR 2 (HY000): file_key_management_filename unset or empty