FieldTransfer: coarsening FE_Nothing cells

[sebproell]

Within #14802 it was discovered that coarsening cells that have FE_Nothing and resolving constraints leads to a floating point exception within FieldTransfer.

How to reproduce

Change the condition in this line

dealii/tests/hp/field_transfer_05.cc

Line 132 in d42cd5f

if (cell->center()[0] < 0.25 && cell->center()[1] < 0.25)

to cell->center()[1] < 0.25