MAGETWO-27267: Add getTierPrices() to the Product

 -- updated per code review
 -- fix API functional test per new behavior

Author: Mike Weis

app/code/Magento/Catalog/Model/Product.php

@@ -962,7 +962,7 @@ public function getGroupPrice()
     /**
      * Gets list of product group prices
      *
-     * @return \Magento\Catalog\Api\Data\ProductGroupPriceInterface[]
+     * @return \Magento\Catalog\Api\Data\ProductGroupPriceInterface[]|null
      */
     public function getGroupPrices()
     {
@@ -984,7 +984,7 @@ public function setGroupPrices(array $groupPrices = null)
     /**
      * Gets list of product tier prices
      *
-     * @return \Magento\Catalog\Api\Data\ProductTierPriceInterface[]
+     * @return \Magento\Catalog\Api\Data\ProductTierPriceInterface[]|null
      */
     public function getTierPrices()
     {

dev/tests/api-functional/testsuite/Magento/ConfigurableProduct/Api/ConfigurableProductManagementTest.php

@@ -69,7 +69,9 @@ public function testGetVariation()
                         'attribute_code' => 'test_configurable',
                         'value' => $attributeOptionValue
                     ]
-                ]
+                ],
+                'group_prices' => [],
+                'tier_prices' => []
             ]
         ];
         ksort($expectedItems);