Make Api\SearchResults implement Api\SearchResultsInterface

Purpose
==

Make the implementation actually fulfill the contract defined by the interface.

Background
==

The return type hints of almost all the repository implementations of the `getList()`
method indicate a return type of `\Magento\Framework\Api\SearchResultsInterface` or
one of its sub-interfaces.

One example of many is `\Magento\Catalog\Api\ProductRepositoryInterface::getList()`.
The return type hint states it returns `\Magento\Catalog\Api\Data\ProductSearchResultsInterface`,
which in turn extends `\Magento\Framework\Api\SearchResultsInterface`.

However, the concrete implementation preference for that interface is
`\Magento\Framework\Api\SearchResults`, which does not implement
`\Magento\Framework\Api\SearchResultsInterface` (or any interface for that matter).

Reasons for this PR
==

This PR makes `Api\SearchResults` actually implement `Api\SearchResultsInterface`,
forcing all implementations to match the contract.
It also improves developer experience because this is pretty much what should be expected
of the code anyway.
It also makes the preference configuration technically more correct, because the configured
concrete class is guaranteed to at least partially fulfill the contract defined by the
interface.

Author: Vinai

app/code/Magento/Catalog/Api/Data/CategoryAttributeSearchResultsInterface.php

@@ -24,5 +24,5 @@ public function getItems();
      * @param \Magento\Catalog\Api\Data\CategoryAttributeInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 }

app/code/Magento/Catalog/Api/Data/ProductAttributeSearchResultsInterface.php

@@ -24,5 +24,5 @@ public function getItems();
      * @param \Magento\Catalog\Api\Data\ProductAttributeInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 }

app/code/Magento/Catalog/Api/Data/ProductSearchResultsInterface.php

@@ -24,5 +24,5 @@ public function getItems();
      * @param \Magento\Catalog\Api\Data\ProductInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 }

app/code/Magento/CatalogInventory/Api/Data/StockCollectionInterface.php

@@ -30,5 +30,5 @@ public function getItems();
      * @param \Magento\CatalogInventory\Api\Data\StockInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 }

app/code/Magento/CatalogInventory/Api/Data/StockItemCollectionInterface.php

@@ -30,7 +30,7 @@ public function getItems();
      * @param \Magento\CatalogInventory\Api\Data\StockItemInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 
     /**
      * Get search criteria.

app/code/Magento/CatalogInventory/Api/Data/StockStatusCollectionInterface.php

@@ -27,7 +27,7 @@ public function getItems();
      * @param \Magento\CatalogInventory\Api\Data\StockStatusInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 
     /**
      * Get search criteria.

app/code/Magento/Cms/Api/Data/BlockSearchResultsInterface.php

@@ -26,5 +26,5 @@ public function getItems();
      * @param \Magento\Cms\Api\Data\BlockInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 }

app/code/Magento/Cms/Api/Data/PageSearchResultsInterface.php

@@ -26,5 +26,5 @@ public function getItems();
      * @param \Magento\Cms\Api\Data\PageInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 }

app/code/Magento/Customer/Api/Data/AddressSearchResultsInterface.php

@@ -26,5 +26,5 @@ public function getItems();
      * @param \Magento\Customer\Api\Data\AddressInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 }

app/code/Magento/Customer/Api/Data/CustomerSearchResultsInterface.php

@@ -26,5 +26,5 @@ public function getItems();
      * @param \Magento\Customer\Api\Data\CustomerInterface[] $items
      * @return $this
      */
-    public function setItems(array $items = null);
+    public function setItems(array $items);
 }