replace_nulls properly propagates memory resource to gather calls

[robertmaynard]

As noted in https://github.com/rapidsai/cudf/pull/8483/files/48b4d97824a6dffd50fe795f698bc1876575f6c2#r650155797 the original code forgot to propagate the device_memory_resource to the detail::gather call.

[codecov]

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[vuule]

@gpucibot merge

[robertmaynard]

rerun tests

[robertmaynard]

rerun tests