Treat a False mandatory variable as defined.

lib/ansible/runner/filter_plugins/core.py

@@ -76,8 +76,11 @@ def skipped(*a, **kw):
 
 def mandatory(a):
     ''' Make a variable mandatory '''
-    if not a:
+    try:
+        a
+    except NameError:
         raise errors.AnsibleFilterError('Mandatory variable not defined.')
+    else:
         return a
 
 def bool(a):