Silence warnings from Email::Address::XS when validating

Do not immediately evaluate `$addr` to prevent `Argument contains empty
address` warnings from Email::Address::XS; instead, use
`$addr->is_valid`.

lib/SVN/Notify.pm

@@ -1476,7 +1476,7 @@ sub output_headers {
         $norm = sub {
             return join ', ' => map {
                 my ($addr) = Email::Address::XS->parse($_);
-                if ($addr) {
+                if ($addr->is_valid) {
                     if (my $phrase = $addr->phrase) {
                         $addr->phrase(Encode::encode( 'MIME-Q', $phrase ));
                     }