Silence Perl warning

Now that warnings are enabled across the board, this code that tries to
print an undef variable emits one.  Silently printing the empty string
achieves the previous behavior.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
Discussion: https://postgr.es/m/E1jO1VT-0008Qk-TM@gemulon.postgresql.org

doc/src/sgml/mk_feature_tables.pl

@@ -54,7 +54,9 @@ while (<$feat>)
 	{
 		print "  <entry>$feature_id</entry>\n";
 	}
-	print "  <entry>" . $feature_packages{$feature_id} . "</entry>\n";
+	print "  <entry>",
+		  defined($feature_packages{$feature_id}) ? $feature_packages{$feature_id} : "",
+		  "</entry>\n";
 	if ($subfeature_id)
 	{
 		print "  <entry>$subfeature_name</entry>\n";