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
@WillAyd @simonjayhawkins I added a follow-up issue and will come back to it later. Otherwise I think I addressed all comments, so this should be good to merge Thanks @SaturnFromTitan  Generally fixes for flaky tests should be backported. Does the 1.0.x branch suffer from the original flakiness? no. #32281 was not backported. Great, thanks.