python: do not crash when a function returns a null formula

* wrap/python/spot_impl.i: Map null formulas to None.
* wrap/python/tests/randgen.py: New file.
* wrap/python/tests/Makefile.am: Add it.

wrap/python/tests/Makefile.am

@@ -48,6 +48,7 @@ TESTS = \
   parsetgba.py \
   piperead.ipynb \
   randaut.ipynb \
+  randgen.py \
   randltl.ipynb \
   relabel.py \
   setxor.py \

wrap/python/tests/randgen.py

@@ -0,0 +1,29 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2015 Laboratoire de Recherche et Développement de
+# l'Epita (LRDE).
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This is a python translation of the ltl2tgba C++ test program.
+# Compare with src/tgbatest/ltl2tgba.cc.
+
+import spot
+
+o = spot.option_map()
+g = spot.randltlgenerator(0, o)
+assert str(g.next()) == '1'
+assert str(g.next()) == '0'
+assert str(g.next()) == 'None'