Tweak handling of empty params when generating report

not sure there was a compelling reason to use `colander.null` other
than that is what pyramid generally does?  but `None` seems to work
fine for me so far..  (used w/ optional date param)

tailbone/views/reports.py

@@ -415,7 +415,7 @@ class GenerateReport(View):
 
             # allow empty value if param is optional
             if not param.required:
-                node.missing = colander.null
+                node.missing = None
 
             # maybe set default value
             if hasattr(param, 'default'):