allow variables to have the value 0

fixes #8155

Test plan:
  * create a formula quiz question
  * make the variable definition x with a min/max of 0
  * make a formula definition that uses x (ex: 5 + x).  You should see a
    computed value, not an error message

Change-Id: Ida604c4b0c10b2567ab6061e8205c823366c9aea
Reviewed-on: https://gerrit.instructure.com/13127
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>

public/javascripts/calcCmd.js

@@ -266,7 +266,7 @@ define([
         }
         var value = predefinedVariables && predefinedVariables[tree.value];
         value = value || (variables && variables[tree.value]);
-        if(!value) {
+        if (value == undefined) {
           throw("undefined variable " + tree.value);
         }
         return value;