If migratewallet fails, we do a cleanup which removes the watchonly and
solvables wallets if they were created. However, if they were not, their
pointers are nullptr and we don't check for that, which causes a
segfault during the cleanup. So check that they aren't nullptr before
cleaning them up.
Github-Pull: #26594
Rebased-From: 86ef7b3c7be84e4183098f448c77ecc9ea7367ab
7a97a56ffb