Ensure the shader exists before attempting to draw

Summary: The shader doesnt necessarily exist when drawing the drawable, so we add a check for that

Differential Revision: D8789279

fbshipit-source-id: f8393d1d3022d4268a37f13318e7c73f52b351af

shimmer/src/main/java/com/facebook/shimmer/ShimmerDrawable.java

@@ -88,7 +88,7 @@ public void onBoundsChange(Rect bounds) {
 
   @Override
   public void draw(@NonNull Canvas canvas) {
-    if (mShimmer == null) {
+    if (mShimmer == null || mShimmerPaint.getShader() == null) {
       return;
     }