Validate project.organization_id NOT NULL constraint

Follow up for !173090 (merged) which added an invalid NOT NULL constraint and was scheduled asynchronous validation.

After the async validation is complete, we need to validate the constraint as specified in https://docs.gitlab.com/ee/development/database/not_null_constraints.html#not-null-constraints-on-large-tables

3. Optional. If the constraint was validated asynchronously, validate the NOT NULL constraint once validation is complete:

# db/post_migrate/
class ValidateMergeRequestDiffsProjectIdNullConstraint < Gitlab::Database::Migration[2.2]
  milestone '16.10'

  def up
    validate_not_null_constraint :merge_request_diffs, :project_id
  end

  def down
    # no-op
  end
end