bug in the sum BlockLinearOperator + ZeroOperator

[e-moral-sanchez]

When a BlockLinearOperator with at least one block that is not a ZeroOperator is summed with a ZeroOperator, the following error appears:

'ZeroOperator' object has no attribute 'set_backend'
  File "/psydac/psydac/linalg/block.py", line 1257, in set_backend
    b.set_backend(backend)

With the inverse order (summing ZeroOperator + BlockLinearOperator) there is no error.

[e-moral-sanchez]

Minimal example:

from sympde.topology                import Cube, Derham, elements_of
from sympde.calculus                import dot
from sympde.expr                    import integral, BilinearForm
from psydac.api.discretization      import discretize
from psydac.api.settings            import PSYDAC_BACKENDS
from psydac.linalg.utilities import array_to_psydac
from psydac.linalg.block import BlockLinearOperator, BlockVector, BlockVectorSpace
from psydac.linalg.basic import IdentityOperator, ZeroOperator

domain = Cube()
derham = Derham(domain)

domain_h = discretize(domain, ncells=[5,5,5], periodic=[False, False, False], comm=None)
derham_h = discretize(derham, domain_h, degree=[1,1,1])
        

E, F = elements_of(derham.V1, names='E, F')
m1 = BilinearForm((E, F), integral(domain, dot(E, F)))
M1 = discretize(m1, domain_h, (derham_h.V1, derham_h.V1), backend=PSYDAC_BACKENDS['pyccel-gcc']).assemble()

V1xV1_h_vector_space = BlockVectorSpace(derham_h.V1.vector_space, derham_h.V1.vector_space)

Z1 = ZeroOperator(derham_h.V1.vector_space, derham_h.V1.vector_space)

op1 = BlockLinearOperator(V1xV1_h_vector_space, V1xV1_h_vector_space, 
                                                blocks=[[Z1, Z1], [Z1,  Z1 + M1]]) #passes

op2 = BlockLinearOperator(V1xV1_h_vector_space, V1xV1_h_vector_space, 
                                                blocks=[[Z1, Z1], [Z1,  M1 +  Z1]]) #fails