ttf fonts: default-family-map: reutrn nil when there is no ttf path
ttf fonts: default-family-map: reutrn nil when there is no ttf path

Currently we hardcode known locations of where to look for fonts in a
variable *TRUETYPE-FONT-PATH*. On some platforms none of these
locations is present -- instead of erroring on passing NIL to the
MERGE-PATHNAMES exit early and allow other methods to trigger the
search.

slyrus/mcclim,slyrus/mcclim