don't 500 when passing nothing into user_search

test plan
 - specs should pass

fixes VICE-2057
flag=none

Change-Id: Ieafc0fbf2e0665ee0fb8ba89359329e14f18a2c6
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/273090
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Drake Harper <drake.harper@instructure.com>
QA-Review: Drake Harper <drake.harper@instructure.com>
Product-Review: Drake Harper <drake.harper@instructure.com>

lib/user_list.rb

@@ -53,6 +53,7 @@ class UserList
     @initial_type = initial_type
     @search_method = (@root_account.open_registration? ? :open : :closed) if @search_method == :infer
     @current_user = current_user
+    list_in ||= ''
     parse_list(list_in)
     resolve
   end

spec/lib/user_list_spec.rb

@@ -40,6 +40,12 @@ describe UserList do
     expect(ul.addresses.map{|x| [x[:name], x[:address]]}).to eq [["senor molé", "blah@instructure.com"]]
   end
 
+  it "should not cause error with nothing past to it" do
+    ul = UserList.new(nil)
+    expect(ul.errors).to eq []
+    expect(ul.addresses.map{|x| [x[:name], x[:address]]}).to eq []
+  end
+
   it "should find by SMS number" do
     user_with_pseudonym(:name => "JT", :active_all => 1)
     communication_channel(@user, {username: '8015555555@txt.att.net', path_type: 'sms', active_cc: true})