"Field is required" when submitting a multiple-value select with only disallowed terms

(This issue occurs with #1214472: Undefined index: und in taxonomy_access_form_alter() -- incorrect assignment of language key applied).

Apparently some form validation for required fields happens before our validation callbacks are used.

Steps to reproduce

1. Create a vocabulary "foo" with terms "bar" and "baz".
2. Add a term reference field to a content type:
   • Select widget
   • Required
   • 2 allowed values
3. Give the authenticated role the following TAC grants:
   • foo vocabulary default: A/A/I/Y/Y
   • bar: A/A/I/N/Y
4. While logged in as an administrator, create a node tagged with "bar".
5. Log in as a regular user. Edit the node body without changing the taxonomy field, and attempt to submit the node

Expected result

The node remains tagged with "bar" and no other terms.

Actual result

The node fails validation with error "Foo field is required"