Fix test for Ruby version of sum method

In Rails 7.1 the `sum` method no longer overrides `Enumerable#sum`. One change
is that there is no implicit conversion of nil to integer. When a fee is
cleared with `Fee::BaseFee#clear` the amount is set to nil and so
claim.basic_fees.sum(&:amount) fails.

It might be appropriate to change `Fee::BaseFee#clear` so that the values
are set to zero instead of nil. However, this may have other side-effects.

spec/services/cleaners/advocate_claim_cleaner_spec.rb

@@ -16,7 +16,7 @@
 RSpec.shared_examples 'clear basic fees' do
   it { expect { call_cleaner }.not_to change { claim.basic_fees.size } }
   it { expect { call_cleaner }.to change { claim.basic_fees.flat_map(&:dates_attended).size }.to 0 }
-  it { expect { call_cleaner }.to change { claim.basic_fees.sum(&:amount) }.to 0 }
+  it { expect { call_cleaner }.to change { claim.basic_fees.sum { |fee| fee.amount.to_i } }.to 0 }
 end
 
 RSpec.shared_examples 'does not clear basic fees' do