Ensure _.add and _.subtract return 0 when no arguments are given.

lodash.js

@@ -13862,6 +13862,9 @@
      */
     function add(augend, addend) {
       var result;
+      if (augend === undefined && addend === undefined) {
+        return 0;
+      }
       if (augend !== undefined) {
         result = augend;
       }
@@ -14072,6 +14075,9 @@
      */
     function subtract(minuend, subtrahend) {
       var result;
+      if (minuend === undefined && subtrahend === undefined) {
+        return 0;
+      }
       if (minuend !== undefined) {
         result = minuend;
       }

test/test.js

@@ -997,6 +997,12 @@
       assert.strictEqual(_.add(-6, -4), -10);
     });
 
+    QUnit.test('should return `0` when no arguments are given', function(assert) {
+      assert.expect(1);
+
+      assert.strictEqual(_.add(), 0);
+    });
+
     QUnit.test('should not coerce arguments to numbers', function(assert) {
       assert.expect(2);
 
@@ -19196,6 +19202,12 @@
       assert.strictEqual(_.subtract(-6, -4), -2);
     });
 
+    QUnit.test('should return `0` when no arguments are given', function(assert) {
+      assert.expect(1);
+
+      assert.strictEqual(_.subtract(), 0);
+    });
+
     QUnit.test('should coerce arguments only numbers', function(assert) {
       assert.expect(2);