Adds validation of email present in valid_email? method

spec/lib/email_digests_spec.rb

@@ -152,6 +152,14 @@ describe EmailDigest do
       email_digest = described_class.new(user)
       expect(email_digest.valid_email?).to be(nil)
     end
+
+    it "returns false if email does not exist" do
+      user = create(:user)
+      user.update_attribute(:email, nil)
+
+      email_digest = described_class.new(user)
+      expect(email_digest.valid_email?).to be(false)
+    end
   end
 
 end