Remove validations in investment changelog entries

If we validate the presence of the old value and the new value, changes
in optional fields will not be stored if either the old value or the new
value are blank.

app/models/budget/investment.rb

@@ -413,7 +413,7 @@ class Budget
             log.investment_id = self.id
             log.new_value = self.send field
             log.old_value = self.send "#{field}_was"
-            !log.save
+            log.save!
           end
         end
       end

app/models/budget/investment/change_log.rb

@@ -5,8 +5,6 @@ class Budget::Investment::ChangeLog < ApplicationRecord
     inverse_of:  :budget_investment_change_logs,
     required:    false
 
-  validates :old_value, presence: true
-  validates :new_value, presence: true
   validates :field, presence: true
 
   scope :by_investment, ->(investment_id) { where(investment_id: investment_id) }