I'm not aware of any studies that have investigated the connection between the Hunga Tonga (or any volcanic) eruption and atmospheric river activity, but I doubt it had any impact on last winter's West Coast activity since the eruption occurred much earlier than last winter's atmospheric rivers. That's not to say that volcanic eruptions cannot leave a longer-lasting impact on the climate, including on ENSO (see here and here), but I do not think the Hunga Tonga eruption was sufficiently explosive for such an impact.

Regarding the impact of El Nino on atmospheric rivers, this previous blog post suggests that activity tends to be elevated in the western US during El Nino winters. So, we may expect an elevated probability of more frequent than normal atmospheric river activity, but as we saw last winter, sometimes the climate system has other plans.