Updated the test of the deleteDescendants to match the consistent ret…

…urn value type integer.

test/testsuite/generator/behavior/nestedset/NestedSetBehaviorObjectBuilderModifierTest.php

@@ -1433,7 +1433,7 @@ public function testDeleteDescendants()
                |  \
                t6 t7
         */
-        $this->assertNull($t2->deleteDescendants(), 'deleteDescendants() returns null leafs');
+        $this->assertEquals(0, $t2->deleteDescendants(), 'deleteDescendants() returns 0 leafs');
         $this->assertEquals(4, $t3->deleteDescendants(), 'deleteDescendants() returns the number of deleted nodes');
         $this->assertEquals(5, $t3->getRightValue(), 'deleteDescendants() updates the current node');
         $this->assertEquals(5, $t4->getLeftValue(), 'deleteDescendants() does not update existing nodes (because delete() clears the instance cache)');