video: console: Fix default font selection

Some callers expect to call this with NULL font name to select the
default font (eg. boot/scene.c). Without handling the NULL condition
U-Boot crashes instead of displaying a bootflow GUI menu.

Signed-off-by: Ondrej Jirman <megi@xff.cz>
Cc: Anatolij Gustschin <agust@denx.de>