Switch criterion for resetting sparsity by
assember from A.size(0) == 0 to A.empty().

Fix implementations of GenericMatrix::empty().

Ensure that GenericMatrix::compressed() does
not silently pass without setting values for
targets without sparsity pattern.
Deprecate AssemblerBase::reset_sparsity.
Remove remnants of AssemblerBase::reset_sparsity.
Merge branch 'blechta/fix-issue-280'
Merge branch 'blechta/fix-reset-sparsity-criterion'