send-email: squelch warning due to comparing undefined $_ to ""

The check to see if initial_reply_to is defined was also comparing $_ to
"" for a reason I cannot ascertain (looking at the commit which made the
change didn't provide enlightenment), but if $_ is undefined, perl
generates a warning.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -406,7 +406,7 @@ if ($thread && !defined $initial_reply_to && $prompting) {
 
 	$initial_reply_to = $_;
 }
-if (defined $initial_reply_to && $_ ne "") {
+if (defined $initial_reply_to) {
 	$initial_reply_to =~ s/^\s*<?/</;
 	$initial_reply_to =~ s/>?\s*$/>/;
 }