Since we now have multiple chainstate objects, only one of them is active at any given time. An active chainstate has a mempool, but there's no point to others having one.
Partial backport of core#22415:
https://github.com/bitcoin/bitcoin/pull/22415/commits/617661703ac29e0744f21de74501d033fdc53ff6#diff-1b75f2d827c545064a075d3a1a1414ddcf11edef153a1bd0e103adcc80c8e219
Also includes a missing null pointer check from D9780 in GetCoinsCacheSizeState().