bounding_box() for t1 driver fonts was treating spaces as empty space

bounding_box() for t1 driver fonts was treating spaces as empty space
when calculating the bounds, which they are, but this is inconsistent
with other drivers.  This was especially a problem for strings
containing only spaces where the left bound would end up far to the
right of the right bound, and similarly for the ascent and descent.

calculating the bounding box for an empty string invoked special
behaviour in t1lib, making it unreliable.

Fixes: http://rt.cpan.org/Ticket/Display.html?id=20555