Fix normalization of BatchInvitationUser#email

Trello: https://trello.com/c/aO4nVCZT

Having implemented strip_whitespace_from_organisation_slug I realised
I'd probably missed a scenario for strip_whitespace_from_email. And sure
enough there was no test coverage for the case when
BatchInvitationUser#email is nil.

In this commit I've improved the test coverage for email validation
which for one thing explicitly forces the presence validation to be
enabled for the email attribute, and for another, it forces me to use
the safe navigation operator in strip_whitespace_from_email. This is
because the latter is called from a before_validation callback and so
email could be nil at this point.

app/models/batch_invitation_user.rb

@@ -103,7 +103,7 @@ def strip_whitespace_from_name
   end
 
   def strip_whitespace_from_email
-    email.strip!
+    email&.strip!
   end
 
   def strip_whitespace_from_organisation_slug

test/models/batch_invitation_user_test.rb

@@ -23,19 +23,25 @@ class BatchInvitationUserTest < ActiveSupport::TestCase
   end
 
   context "validations" do
-    should "validate email address" do
+    should "validate presence of email address" do
+      user = build(:batch_invitation_user, email: nil)
+
+      assert_not user.valid?
+      assert_includes user.errors[:email], "can't be blank"
+    end
+
+    should "validate format of email address" do
       user = build(:batch_invitation_user, email: "@gov.uk")
 
       assert_not user.valid?
-      assert_equal ["is invalid"], user.errors[:email]
+      assert_includes user.errors[:email], "is invalid"
     end
 
     should "prevent user being created with a known non-government email address" do
       user = build(:batch_invitation_user, email: "piers.quinn@yahoo.co.uk")
 
       assert_not user.valid?
-      assert_equal ["not accepted. Please enter a workplace email to continue."],
-                   user.errors[:email]
+      assert_includes user.errors[:email], "not accepted. Please enter a workplace email to continue."
     end
 
     should "not allow user to be updated with a known non-government email address" do