misc: Fix some warning in specs

[vincent-pochet]

This PR fixes the following warning messages when running the specs:

WARNING: An expectation of `:should_sync_invoice?` was set on `nil`. To allow expectations on `nil` and suppress this message, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `true`. To disallow expectations on `nil`, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `false`. Called from /home/runner/work/lago-api/lago-api/spec/services/invoices/finalize_open_credit_service_spec.rb:12:in `block (2 levels) in <top (required)>'.

WARNING: An expectation of `:should_sync_sales_order?` was set on `nil`. To allow expectations on `nil` and suppress this message, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `true`. To disallow expectations on `nil`, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `false`. Called from /home/runner/work/lago-api/lago-api/spec/services/invoices/finalize_open_credit_service_spec.rb:13:in `block (2 levels) in <top (required)>'.

[nudded]

ah nice, happy to get rid of these!