Simplify i9e_line_handler.

The line handlers of all three users of the i9e API (play, client
and audioc) return immediately if the passed line is NULL or the
empty string. Hence we may call the line handler only if there is a
non-empty line to dispatch.

Moving the check for a non-empty line to generic i9e code simplifies
the three line handlers a bit and avoids code duplication.