fix creating user with no sis_user_id. fixes #6872

admin can now create user with no sis_user_id without receiving
an error about the sis id already existing.

also includes a migration to set all empty string sis_user_ids to
NULL.

test plan:
  * enable sis imports on an account;
  * as an admin in that account, create a new user, leaving the
    sis id blank;
  * create a second new user, also with a blank sis id;
  * verify that the second user is creating without any errors.

test plan:

Change-Id: Ia247c58e0070a4b02ecc584c18ed20bb5c05dcd3
Reviewed-on: https://gerrit.instructure.com/8665
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>

app/models/pseudonym.rb

@@ -133,6 +133,7 @@ class Pseudonym < ActiveRecord::Base
     if !crypted_password || crypted_password == ""
       self.generate_temporary_password
     end
+    self.sis_user_id = nil if self.sis_user_id.blank?
   end
   
   def update_passwords_on_related_pseudonyms

db/migrate/20120216214454_set_blank_sis_user_ids_to_null.rb

@@ -0,0 +1,9 @@
+class SetBlankSisUserIdsToNull < ActiveRecord::Migration
+  def self.up
+    Pseudonym.update_all({ :sis_user_id => nil }, :sis_user_id => '')
+  end
+
+  def self.down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

spec/models/pseudonym_spec.rb

@@ -135,6 +135,15 @@ describe Pseudonym do
     @pseudonym.should be_deleted
   end
 
+  it "should change a blank sis_user_id to nil" do
+    user
+    pseudonym = Pseudonym.new(:user => @user, :unique_id => 'test@example.com', :password => 'pwd123')
+    pseudonym.password_confirmation = 'pwd123'
+    pseudonym.sis_user_id = ''
+    pseudonym.should be_valid
+    pseudonym.sis_user_id.should be_nil
+  end
+
   it "should gracefully handle unreachable LDAP servers" do
     require 'net/ldap'
     module Net