fix usage of the ACTNUM keyword exported by EclipseGrid

it turns out that EclipseGrid::exportACTNUM() produces a vector of
zero length instead of an all-1 one of the size of the logically
Cartesian grid if ACTNUM is not set.

Strangely enough, this worked if libstdc++'s debug mode was not
enabled. (maybe because dune-cornerpoint does not consider ACTNUM?)

@akva2: does this fix the recent opm-upscaling issues?