As we've improved the cty.Value normalization, we need to remove
normalization procedures from the flatmap handling. Keeping the empty
containers in the flatmap will prevent unexpected nils from being added
to some schema configurations
The NewExtra values are stored outside the diff from plan, and the
original keys may not contain the ~ prefix. Adding the NewExtra back
into the diff with the mismatched key was causing an entire new set
element to be populated. Since this symbol isn't used to apply the diff
in helper/schema, we can simply strip them out.