Fix textShadowOffset error without width or height

Summary:
textShadowOffset design is `ReactPropTypes.shape({width: ReactPropTypes.number, height: ReactPropTypes.number})`, so either width or height is optional.
Unfortunately, in Android implementation, it is my bad not handling optional case and lead to an exception.
Thanks kohver for reporting [this issue](#4975 (comment))

**Test plan (required)**

*Before this fix*
1. Modify TextExample.android.js to `<Text style={{fontSize: 20, textShadowOffset: {height: 10}, textShadowRadius: 1, textShadowColor: '#00cccc'}}>` which really raise a redbox.

*After this fix*
1. Test original TextExample.android.js textShadowOffset works well.
2. Modify TextExample.android.js to `<Text style={{fontSize: 20, textShadowOffset: {height: 10}, textShadowRadius: 1, textShadowColor: '#00cccc'}}>` which works well without redbox.
Closes #7119

Differential Revision: D3240607

Pulled By: mkonicek

fb-gh-sync-id: b13221ae1586594890b0f4aa644ace7c0d5d6c58
fbshipit-source-id: b13221ae1586594890b0f4aa644ace7c0d5d6c58

Author: Kudo

ReactAndroid/src/main/java/com/facebook/react/views/text/ReactTextShadowNode.java

@@ -67,8 +67,11 @@ public class ReactTextShadowNode extends LayoutShadowNode {
   public static final String PROP_TEXT = "text";
 
   public static final String PROP_SHADOW_OFFSET = "textShadowOffset";
+  public static final String PROP_SHADOW_OFFSET_WIDTH = "width";
+  public static final String PROP_SHADOW_OFFSET_HEIGHT = "height";
   public static final String PROP_SHADOW_RADIUS = "textShadowRadius";
   public static final String PROP_SHADOW_COLOR = "textShadowColor";
+
   public static final int DEFAULT_TEXT_SHADOW_COLOR = 0x55000000;
 
   private static final TextPaint sTextPaintInstance = new TextPaint();
@@ -474,13 +477,22 @@ public void setTextDecorationLine(@Nullable String textDecorationLineString) {
 
   @ReactProp(name = PROP_SHADOW_OFFSET)
   public void setTextShadowOffset(ReadableMap offsetMap) {
-    if (offsetMap == null) {
-      mTextShadowOffsetDx = 0;
-      mTextShadowOffsetDy = 0;
-    } else {
-      mTextShadowOffsetDx = PixelUtil.toPixelFromDIP(offsetMap.getDouble("width"));
-      mTextShadowOffsetDy = PixelUtil.toPixelFromDIP(offsetMap.getDouble("height"));
+    mTextShadowOffsetDx = 0;
+    mTextShadowOffsetDy = 0;
+
+    if (offsetMap != null) {
+      if (offsetMap.hasKey(PROP_SHADOW_OFFSET_WIDTH) &&
+          !offsetMap.isNull(PROP_SHADOW_OFFSET_WIDTH)) {
+        mTextShadowOffsetDx =
+            PixelUtil.toPixelFromDIP(offsetMap.getDouble(PROP_SHADOW_OFFSET_WIDTH));
+      }
+      if (offsetMap.hasKey(PROP_SHADOW_OFFSET_HEIGHT) &&
+          !offsetMap.isNull(PROP_SHADOW_OFFSET_HEIGHT)) {
+        mTextShadowOffsetDy =
+            PixelUtil.toPixelFromDIP(offsetMap.getDouble(PROP_SHADOW_OFFSET_HEIGHT));
+      }
     }
+
     markUpdated();
   }