Do not output empty parse error blocks in the CGI script.

* wrap/python/spot.i: Provide a __nonzero__() method for
parse_error_list.
* wrap/python/cgi-bin/ltl2tgba.in: Do not call format_parse_errors()
unconditionally.

wrap/python/spot.i

@@ -280,3 +280,13 @@ unblock_signal(int signum)
 }
 
 %}
+
+%extend spot::ltl::parse_error_list {
+
+bool
+__nonzero__()
+{
+  return !self->empty();
+}
+
+}