[Flow] Ignore validation errors for related object

[Stephen Bungert]

Hi steffen,

my setter also accepts NULL.

I have found a solution, it is to rename the fields that start with 
book[image], if the use clicks the add image button in my app I then 
rename them back so that an image object is created.

If they don't then there are no book[image] fields submitted and flow 
doesn't try and create an image object.

I'm sure there must be a better way.