Mk/Uses/magick.mk: Fix DEFAULT_VERSION handling

Handle fallback for version and flavor separately, also add several
sanity checks.

This fixes creating broken *_DEPENDS for ports using it without
arguments when a -nox11 version is requested in DEFAULT_VERSIONS like
e.g. this resulting from "imagemagick=7-nox11":
libMagick++-7-nox11.so:graphics/ImageMagick7-nox11

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D39424