macosx: advanced prefs: save the default value if Default is selected, not NULL
macosx: advanced prefs: save the default value if Default is selected, not NULL

fixes #8942. Default value for avcodec-hw is none, but NULL was saved, so
hardware decoding got enabled unintentionally.
(cherry picked from commit 3a96bb0f05cdcd427d53333b9461d00fa23060c2)

Signed-off-by: David Fuhrmann <1118fa3e4e403c0aeed361cc3f3eb9aca4cc425c@googlemail.com>

jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1