builtin-commit.c: guard config parser from value=NULL
builtin-commit.c: guard config parser from value=NULL

commit.template configuration expects a string value.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git