default hitSlop values to 0 (#22281)

Summary:
Experienced a `TouchableOpacity` releasing the `PanResponder` without invoking `onPress` due to a missing a direction key in the `hitSlop` prop; The missing key caused the corresponding pressExpand to become NaN which causes `isTouchWithinActive` to become falsey, when it should be truthy.

If defaulting to 0 is undesired behavior, I'm happy to take a different approach.
Pull Request resolved: #22281

Differential Revision: D13374335

Pulled By: cpojer

fbshipit-source-id: f9d28e51b9d9c45aed42bea2df3d844a799fa827

Libraries/Components/Touchable/Touchable.js

@@ -480,10 +480,10 @@ const TouchableMixin = {
       : null;
 
     if (hitSlop) {
-      pressExpandLeft += hitSlop.left;
-      pressExpandTop += hitSlop.top;
-      pressExpandRight += hitSlop.right;
-      pressExpandBottom += hitSlop.bottom;
+      pressExpandLeft += hitSlop.left || 0;
+      pressExpandTop += hitSlop.top || 0;
+      pressExpandRight += hitSlop.right || 0;
+      pressExpandBottom += hitSlop.bottom || 0;
     }
 
     const touch = TouchEventUtils.extractSingleTouch(e.nativeEvent);