Possible null pointer dereference due to return value of folder.listFiles() in WebModuleUtil

On line  https://github.com/openmrs/openmrs-core/blob/master/web/src/main/java/org/openmrs/module/web/WebModuleUtil.java#L687 

you will get a null pointer exception if folder.listFiles() returns null. We therefore need to check for null before using the return value.