A patch from Taher Alkhateeb for "Corrections and improvements to dependent drop-downs" https://issues.apache.org/jira/browse/OFBIZ-5126

* The getDependentDropdownValues function in miscAjaxFunctions.js is modified with a new argument to allow for empty selections. Therefore, copying the behavior of form widget drop-down's "allow-empty"
* There is only one file affected by item number 2 above: setPriceRulesCondEventJs.ftl Therefore, I fixed it to include the argument in getDependentDropdownValues

jleroux: complete r1438501 where I missed setPriceRulesCondEventJs.ftl, as suggested Taher, for consistency sake, I used '' instead of false

git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1438876 13f79535-47bb-0310-9956-ffa450edef68