notes.c: fix a segfault in notes_display_config()

notes.c: fix a segfault in notes_display_config()

If notes.displayRef is configured with no value[1], control should be
returned to the caller when notes.c:notes_display_config() checks if 'v'
is NULL. Otherwise, both git log --notes and git diff-tree --notes will
subsequently segfault when refs.h:has_glob_specials() calls strpbrk()
with a NULL first argument.

[1] Examples:
.git/config:
[notes]
	displayRef
$ git -c notes.displayRef [...]

Signed-off-by: Nate Avers <nate@roosteregg.cc>
Signed-off-by: Junio C Hamano <gitster@pobox.com>