Fabric: Unification of ShadowView's default constructor

Summary:
The previous version of a set of default values of `ShadowView`'s fields has a bug:
```
  ComponentName componentName = "";
```

Initalizing `char const *` with a string literal in .h file makes the default constructor of the object produces different values across binary units (because a pointer to empty string can be defined differently). Now it's just a null pointer.

Reviewed By: sammy-SC

Differential Revision: D15911452

fbshipit-source-id: 16bcfb5f78ea802c0833135c486e3fbb8b7acaa6

ReactCommon/fabric/mounting/ShadowView.h

@@ -22,30 +22,28 @@ namespace react {
  */
 struct ShadowView final {
   ShadowView() = default;
-  ShadowView(const ShadowView &shadowView) = default;
+  ShadowView(ShadowView const &shadowView) = default;
   ShadowView(ShadowView &&shadowView) noexcept = default;
 
-  ~ShadowView(){};
-
   /*
    * Constructs a `ShadowView` from given `ShadowNode`.
    */
-  explicit ShadowView(const ShadowNode &shadowNode);
+  explicit ShadowView(ShadowNode const &shadowNode);
 
-  ShadowView &operator=(const ShadowView &other) = default;
+  ShadowView &operator=(ShadowView const &other) = default;
   ShadowView &operator=(ShadowView &&other) = default;
 
-  bool operator==(const ShadowView &rhs) const;
-  bool operator!=(const ShadowView &rhs) const;
+  bool operator==(ShadowView const &rhs) const;
+  bool operator!=(ShadowView const &rhs) const;
 
-  ComponentName componentName = "";
-  ComponentHandle componentHandle = 0;
-  Tag tag = -1; // Tag does not change during the lifetime of a shadow view.
-  SharedProps props = {};
-  SharedEventEmitter eventEmitter = {};
-  LayoutMetrics layoutMetrics = EmptyLayoutMetrics;
-  SharedLocalData localData = {};
-  State::Shared state = {};
+  ComponentName componentName{};
+  ComponentHandle componentHandle{};
+  Tag tag{};
+  Props::Shared props{};
+  EventEmitter::Shared eventEmitter{};
+  LayoutMetrics layoutMetrics{EmptyLayoutMetrics};
+  LocalData::Shared localData{};
+  State::Shared state{};
 };
 
 #if RN_DEBUG_STRING_CONVERTIBLE