Some interpolators fail with zero-sized target functionspace.

[odlomax]

Sometimes, when performing an interpolation, situations arise where one or more partitions have zero-sized target functionspaces.

As reported in jcsda oops #2664, an error occurs when the target functionspace of the Structured2D interpolation method has zero size. This triggers an error where the eckit::SparseMatrix fails to construct with zero non-zero elements. If we avoid constructing the interpolation object, MPI deadlocks can occur.