CI: Fix flaky test_value_counts_null
As mentioned by @simonjayhawkins in #32438

Trace of the failing test:
https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=29965&view=logs&j=077026cf-93c0-54aa-45e0-9996ba75f6f7&t=e95cf409-86ae-5b4d-6c5f-79395ef75e8f