var: allow GIT_EDITOR to return null

var: allow GIT_EDITOR to return null

The handling to die early when there is no EDITOR is valuable when
used in normal code (i.e., editor.c). In git-var, where
null/empty-string is a perfectly valid value to return, it doesn't
make as much sense.

Remove this handling from `git var GIT_EDITOR` so that it does not
fail so noisily when there is no defined editor.

Signed-off-by: Sean Allred <allred.sean@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>