Clear total charge as well (#1095)

app/models/lettings_log.rb

@@ -555,6 +555,7 @@ private
       self.scharge = nil
       self.pscharge = nil
       self.supcharg = nil
+      self.tcharge = nil
     end
 
     errors.clear

spec/models/lettings_log_spec.rb

@@ -2073,6 +2073,7 @@ RSpec.describe LettingsLog do
           expect(lettings_log.scharge).to eq(nil)
           expect(lettings_log.pscharge).to eq(nil)
           expect(lettings_log.supcharg).to eq(nil)
+          expect(lettings_log.tcharge).to eq(nil)
         end
 
         it "does not impact other validations" do