Creating an interface to check for emptiness of an object

Basically, MobileFrontEnd PHP codebase has a use case where
it would be more efficient to check for emptiness of a collection
rather than counting its elements and comparing it to 0.

bug: T151797
Change-Id: I309a549c82c0bdd80640f509557f2a48278e13ce

includes/Emptiable.php

@@ -0,0 +1,15 @@
+<?php
+namespace MediaWiki;
+
+/**
+ * An interface to check for emptiness of an object
+ */
+interface Emptiable {
+
+	/**
+	 * Check if object is empty
+	 * @return bool Is it empty
+	 */
+	public function isEmpty();
+
+}