Empty level fix (#3517)

## Summary

Closes #3516 . This works for me in practice, and is more convenient
than filtering the levels on the caller's side.

## Additional background

Idea is to simply not consider the empty levels in the linear solver.

Src/LinearSolvers/MLMG/AMReX_MLLinOp.H

@@ -736,7 +736,12 @@ MLLinOpT<MF>::defineGrids (const Vector<Geometry>& a_geom,
 {
     BL_PROFILE("MLLinOp::defineGrids()");
 
-    m_num_amr_levels = static_cast<int>(a_geom.size());
+    m_num_amr_levels = 0;
+    for (int amrlev = 0; amrlev < a_geom.size(); amrlev++) {
+        if (!a_grids[amrlev].empty()) {
+            m_num_amr_levels++;
+        }
+    }
 
     m_amr_ref_ratio.resize(m_num_amr_levels);
     m_num_mg_levels.resize(m_num_amr_levels);