Move stray concern for nilifying blanks

* Was in `lib`, can now live in `Concerns::NilifyBlanks`
* actual genuine concern that autoloads, not the sort-of-concerns we previously moved, so needs to be in a fixed `Concerns` namespace.
How can this be included with a different namespace? `include ActiveRecord::Concerns::NilifyBlanks` Does Rails add it to the ActiveRecord namespace magically?