Fix Display::Subpanel#content length when data_l non-string.

`Display::Subpanel#content` worked when data_l `nil` as well as string.
When non-string, however, `.length` failed. For non-string data_r,
however, everything worked as expected, because of `.to_s` being called
implicitly. Fix by explicitly calling `.to_s`, which will also work for
`nil`.