Update StandardMessageCodec.readValue to be @Nullable

shell/platform/android/io/flutter/plugin/common/StandardMessageCodec.java

@@ -331,7 +331,7 @@ protected static final void readAlignment(@NonNull ByteBuffer buffer, int alignm
   }
 
   /** Reads a value as written by writeValue. */
-  @NonNull
+  @Nullable
   protected final Object readValue(@NonNull ByteBuffer buffer) {
     if (!buffer.hasRemaining()) {
       throw new IllegalArgumentException("Message corrupted");