add: implement isEmpty

Author: qiao

src/immutable_sequence.js

@@ -55,6 +55,14 @@
     return new ImmutableSequence(tree);
   };
 
+  /**
+   * Check whether the sequence is empty.
+   * @return {boolean}
+   */
+  ImmutableSequence.prototype.isEmpty = function () {
+    return this.tree_.isEmpty();
+  };
+
   /**
    * Get the size of the sequence.
    * @return {number}