Merge branch 'stefanozampini/firedrake-1' into 'main'

Updates for Firedrake: first batch

See merge request petsc/petsc!7358

MATNEST: support all NULL blocks

This is useful for doing something like

MatCreateNest(...,nr, isr, nc, isc, NULL, &N);
...
MatNestSetSubMat(N,0,0,...)

Add missing interface checks and tests