Set to enabled on rebuild when options is not empty and disableIfEmpty is true (issue: #494)

dist/js/bootstrap-multiselect.js

@@ -1153,6 +1153,9 @@
             if (this.options.disableIfEmpty && $('option', this.$select).length <= 0) {
                 this.disable();
             }
+            else {
+                this.enable();
+            }
             
             if (this.options.dropRight) {
                 this.$ul.addClass('pull-right');