check for empty string parameters

test plan
* you should be able to pass 'all terms' on a 
report with a term selector

Change-Id: Iaf8eb3c59f2818233477c490b3a030fc24ebfbd4
Reviewed-on: https://gerrit.instructure.com/16953
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Brad Humphrey <brad@instructure.com>

app/models/account_report.rb

@@ -1,5 +1,5 @@
 #
-# Copyright (C) 2011 - 2012 Instructure, Inc.
+# Copyright (C) 2011 - 2013 Instructure, Inc.
 #
 # This file is part of Canvas.
 #
@@ -76,7 +76,7 @@ class AccountReport < ActiveRecord::Base
   handle_asynchronously :run_report
 
   def has_parameter?(key)
-    self.parameters.is_a?(Hash) && self.parameters.has_key?(key)
+    self.parameters.is_a?(Hash) && self.parameters[key].presence
   end
 
   def self.available_reports(account)