Don't bother storing a null value in the cache

It serves very little purpose, and it causes an NPE.

Bug: 9286049
Change-Id: I8c95d8ded2bebdc9b904ef788cbc70865144213e
diff --git a/src/com/android/ex/chips/BaseRecipientAdapter.java b/src/com/android/ex/chips/BaseRecipientAdapter.java
index 71b610e..a9aee46 100644
--- a/src/com/android/ex/chips/BaseRecipientAdapter.java
+++ b/src/com/android/ex/chips/BaseRecipientAdapter.java
@@ -765,8 +765,10 @@
                             mHandler.post(new Runnable() {
                                 @Override
                                 public void run() {
-                                    mPhotoCacheMap.put(photoThumbnailUri, photoBytes);
-                                    notifyDataSetChanged();
+                                    if (photoBytes != null) {
+                                        mPhotoCacheMap.put(photoThumbnailUri, photoBytes);
+                                        notifyDataSetChanged();
+                                    }
                                 }
                             });
                         }