Use a cheaper call.

Use a cheaper call.

The functions in question take an IndexSet argument that can either mean
something, or if it is empty, is ignored. In the places where an empty
IndexSet is passed, it actually has size zero, not just zero elements,
but in the existing code, we test whether 'indices.n_elements()==0'.
It turns out that 'n_elements()' is a pretty expensive function to call.
What we really wanted, of course, was 'indices.size()==0', and this
patch makes that change.

I ran all multigrid tests (the functions in question are multigrid
related), and they all succeed.