send-email: fix uninitialized var warning for $smtp_auth

On the latest version of git-send-email, I see this error just before
running SMTP auth (I didn't provide any --smtp-auth= parameter):

  Use of uninitialized value $smtp_auth in pattern match (m//) at \
  /home/briannorris/git/git/git-send-email.perl line 1139.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1142,7 +1142,7 @@ sub smtp_auth_maybe {
 
 	# Check mechanism naming as defined in:
 	# https://tools.ietf.org/html/rfc4422#page-8
-	if ($smtp_auth !~ /^(\b[A-Z0-9-_]{1,20}\s*)*$/) {
+	if ($smtp_auth && $smtp_auth !~ /^(\b[A-Z0-9-_]{1,20}\s*)*$/) {
 		die "invalid smtp auth: '${smtp_auth}'";
 	}