validate_after_scope_change? use compact before retrieving max

Otherwise produces error "ArgumentError: comparison of NilClass with
String failed" if the resultant contains a combinations of strings and
nils.

NEWS.md

@@ -55,6 +55,9 @@
 * Fix `permit` so that it does not break the functionality of
   ActionController::Parameters#require. ([#648], [#675])
 
+* Fix `validate_uniqueness_of` + `scoped_to` so that it does not raise an error
+  if a record exists where the scoped attribute is nil. ([#677])
+
 ### Features
 
 * Add `on` qualifier to `permit`. This allows you to make an assertion that
@@ -71,6 +74,7 @@
 [#607]: https://github.com/thoughtbot/shoulda-matchers/pull/607
 [#648]: https://github.com/thoughtbot/shoulda-matchers/pull/648
 [#675]: https://github.com/thoughtbot/shoulda-matchers/pull/675
+[#677]: https://github.com/thoughtbot/shoulda-matchers/pull/677
 
 # 2.8.0
 

lib/shoulda/matchers/active_record/validate_uniqueness_of_matcher.rb

@@ -386,7 +386,7 @@ module Shoulda
           else
             all_records = @subject.class.all
             @options[:scopes].all? do |scope|
-              previous_value = all_records.map(&scope).max
+              previous_value = all_records.map(&scope).compact.max
 
               next_value =
                 if previous_value.blank?

spec/unit/shoulda/matchers/active_record/validate_uniqueness_of_matcher_spec.rb

@@ -390,6 +390,17 @@ describe Shoulda::Matchers::ActiveRecord::ValidateUniquenessOfMatcher, type: :mo
           array: true
       end
     end
+
+    context "when an existing record that is not the first has a nil value for the scoped attribute" do
+      it 'still works' do
+        model = define_model_validating_uniqueness(scopes: [:scope])
+        create_record_from(model, scope: 'some value')
+        create_record_from(model, scope: nil)
+        record = build_record_from(model, scope: 'a different value')
+
+        expect(record).to validate_uniqueness.scoped_to(:scope)
+      end
+    end
   end
 
   context 'when the model has a case-sensitive validation' do