Add nullability annotations to PFProductTableViewController.

Author: nlutsenko

ParseUI/Classes/ProductTableViewController/PFProductTableViewController.h

@@ -19,8 +19,13 @@
  *
  */
 
+#import <UIKit/UIKit.h>
+
+#import <ParseUI/ParseUIConstants.h>
 #import <ParseUI/PFQueryTableViewController.h>
 
+PFUI_ASSUME_NONNULL_BEGIN
+
 /*!
  `PFProductTableViewController` displays in-app purchase products stored on Parse.
  In addition to setting up in-app purchases in iTunes Connect, the app developer needs
@@ -38,3 +43,5 @@
 - (instancetype)initWithStyle:(UITableViewStyle)style NS_DESIGNATED_INITIALIZER;
 
 @end
+
+PFUI_ASSUME_NONNULL_END