git-send-email: prevent undefined variable warnings if no encryption is set

With the previous patch, not configuring any encryption (either on or
off) would leave $smtp_encryption undefined.  We simply set it to the
empty string in that case.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -313,6 +313,9 @@ foreach my $setting (values %config_bool_settings) {
 	${$setting->[0]} = $setting->[1] unless (defined (${$setting->[0]}));
 }
 
+# 'default' encryption is none -- this only prevents a warning
+$smtp_encryption = '' unless (defined $smtp_encryption);
+
 # Set CC suppressions
 my(%suppress_cc);
 if (@suppress_cc) {