send-email: minor code refactoring

Group expressions in a single if statement. This avoid checking
multiple time if the variable $sender is defined.

Signed-off-by: Remi Lespinet <remi.lespinet@ensimag.grenoble-inp.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -774,9 +774,9 @@ if (!$force) {
 	}
 }
 
-($sender) = expand_aliases($sender) if defined $sender;
-
-if (!defined $sender) {
+if (defined $sender) {
+	($sender) = expand_aliases($sender);
+} else {
 	$sender = $repoauthor || $repocommitter || '';
 }