This avoids an assert failure when assigning a value to an empty array.

This is necessary to fix build with GCC 10, other branches need to be
rebased ASAP.

To/matrices

See merge request !59

To/matrices

See merge request !58

Otherwise the compiler would complain for unsigned types, see also Array
and Vector methods.

- DistributedMatrixTest is still broken because the rowVectorProduct
  method is not implemented, see issue #65

- BiEllpack and ChunkedEllpack are broken because the "segmentsCount"
  attribute is missing, see issue #67