Disable 'Generate Exception Certificates' button when there are no certificate exceptions.

lms/static/js/certificates/views/certificate_whitelist.js

@@ -32,7 +32,12 @@
                 render: function(){
                     var template = this.loadTemplate('certificate-white-list');
                     this.$el.html(template({certificates: this.collection.models}));
-
+                    if (this.collection.isEmpty()) {
+                        this.$("#generate-exception-certificates").addClass("is-disabled");
+                    }
+                    else {
+                        this.$("#generate-exception-certificates").removeClass("is-disabled");
+                    }
                 },
 
                 loadTemplate: function(name) {