SkFontMgr::matchFamily should not crash on nullptr.

SkFontMgr::matchFamily should not crash on nullptr.

While all systems can resolve a font from just a style request
(without a name) almost no systems specify a default font family.

BUG=skia:6574

Change-Id: If7c81808b62cd5d8212bce2eb4d9c476c45af80a
Reviewed-on: https://skia-review.googlesource.com/14902
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Reed <reed@google.com>