MySQL 5.7 introduced WL#7943: InnoDB: Implement Information_Schema.Files
to provide a long-term alternative for accessing tablespace metadata.
The INFORMATION_SCHEMA.INNODB_* views are considered internal interfaces
that are subject to change or removal between releases. So, users should
refer to I_S.FILES instead of I_S.INNODB_SYS_TABLESPACES to fetch metadata
about CREATE TABLESPACE.

Because MariaDB 10.2 does not support CREATE TABLESPACE or
CREATE TABLE…TABLESPACE for InnoDB, it does not make sense to support
I_S.FILES either. So, let MariaDB 10.2 omit the code that was added in
MySQL 5.7. After this change, I_S.FILES will report the empty result,
unless some other storage engine in MariaDB 10.2 implements the interface.
(The I_S.FILES interface was originally created for the NDB Cluster.)