Fix initialization of `autocreate` and `use_ssl` (#2309)

According to the documentation, both `OBJECTSTORE_S3_SSL` and
`OBJECTSTORE_S3_AUTOCREATE` should default to `true`.
Currently, when these environment variables are not set, they default to
`false`. (See https://github.com/nextcloud/docker/issues/2308

).

This fix works, because `strtolower(false)` returns the empty string. So
when `OBJECTSTORE_S3_SSL` is not set and `getenv('OBJECTSTORE_S3_SSL')`
returns `false`, the check `strtolower($use_ssl) !== 'false'` will
evaluate to `true`.

With this fix, both values will be `true` if they are

* not set
* the empty string
* any string that is not equal to `false` when converted to lowercase

This should now match the documented behavior.

Signed-off-by: Valentin Brandl <mail@vbrandl.net>