Fix extra nil added to assigned investment IDs

When `valuator_group` was `nil`, `[valuator_group&.investment_ids]` is
evaluated to `nil`, and so we were adding an extra element to the array.

We could add a `compact` call to the resulting array, but I find it
easier to convert `nil` to an array using `to_a`.

app/models/valuator.rb

@@ -18,7 +18,7 @@ class Valuator < ApplicationRecord
   end
 
   def assigned_investment_ids
-    investment_ids + [valuator_group&.investment_ids].flatten
+    investment_ids + valuator_group&.investment_ids.to_a
   end
 
 end

spec/models/valuator_spec.rb

@@ -28,8 +28,8 @@ describe Valuator do
       investment2.valuators << valuator
 
       assigned_investment_ids = valuator.assigned_investment_ids
-      expect(assigned_investment_ids).to include investment1.id
-      expect(assigned_investment_ids).to include investment2.id
+
+      expect(assigned_investment_ids).to match_array [investment1.id, investment2.id]
       expect(assigned_investment_ids).not_to include investment3.id
     end
 
@@ -45,8 +45,8 @@ describe Valuator do
       investment2.valuator_groups << group
 
       assigned_investment_ids = valuator.assigned_investment_ids
-      expect(assigned_investment_ids).to include investment1.id
-      expect(assigned_investment_ids).to include investment2.id
+
+      expect(assigned_investment_ids).to match_array [investment1.id, investment2.id]
       expect(assigned_investment_ids).not_to include investment3.id
     end
   end