Fix following of Falsy elements in get_children

The problem can arise if user classes are used which can evaluate to `False` in
boolean contexts.

CHANGELOG.md

@@ -29,6 +29,7 @@ please take a look at related PRs and issues and see if the change affects you.
 
 ### Fixed
 
+  - Fixed bug with Falsy user classes in `get_children` ([#288])
   - Fixed bug with unhashable objects during dot export ([#283])
   - Fixed bug where (Ext)RelativeName scope providers accepted any referenced
     object that contained the lookup name in its name. Thanks ipa-mdl@GitHub
@@ -496,6 +497,7 @@ please take a look at related PRs and issues and see if the change affects you.
   - Export to dot.
 
 
+[#288]: https://github.com/textX/textX/pull/288
 [#284]: https://github.com/textX/textX/pull/284
 [#283]: https://github.com/textX/textX/pull/283
 [#282]: https://github.com/textX/textX/pull/282

textx/model.py

@@ -100,7 +100,7 @@ def follow(elem):
                 if attr.cont:
                     if attr.mult in (MULT_ONE, MULT_OPTIONAL):
                         new_elem = getattr(elem, attr_name)
-                        if new_elem and should_follow(new_elem):
+                        if new_elem is not None and should_follow(new_elem):
                             follow(new_elem)
                     else:
                         new_elem_list = getattr(elem, attr_name)