Treat blank parent email as nil (and valid)

dashboard/app/models/user.rb

@@ -247,6 +247,7 @@ class User < ActiveRecord::Base
 
   has_many :user_geos, -> {order 'updated_at desc'}
 
+  before_validation :normalize_parent_email
   validate :validate_parent_email
 
   after_create :associate_with_potential_pd_enrollments
@@ -2080,6 +2081,10 @@ def get_student_hidden_ids(assign_id, hidden_stages)
     end
   end
 
+  def normalize_parent_email
+    self.parent_email = nil if parent_email.blank?
+  end
+
   # Parent email is not required, but if it is present, it must be a
   # well-formed email address.
   def validate_parent_email

dashboard/test/controllers/registrations_controller/update_test.rb

@@ -223,6 +223,22 @@ class UpdateTest < ActionDispatch::IntegrationTest
       assert_nil student.parent_email
     end
 
+    test "single-auth student can update with a blank parent email without password" do
+      student = create :student, :unmigrated_clever_sso
+      assert_nil student.hashed_email
+      assert_nil student.parent_email
+
+      sign_in student
+      put '/users', params: {
+          format: 'json',
+          user: {parent_email: '', age: '9'}
+      }
+      assert_response :no_content
+
+      student.reload
+      assert_nil student.parent_email
+    end
+
     # The next several tests explore profile changes for users with or without
     # passwords.  Examples of users without passwords are users that authenticate
     # via oauth (a third-party account), or students with a picture password.