grdview: Fix the bug for the default z-plane when only -N is set

[seisman]

The grdview -N docs say (https://docs.generic-mapping-tools.org/6.6/grdview.html#n):

If no level is set then we default to the minimum value in the reliefgrid. However, if -R was used to set zmin/zmax then we use that value if it is less than the grid minimum value.

Currently, when -N is set but level is not given, the plane is not drawn at all. It can be reproduced by the following command:

gmt grdview @earth_relief_01d -R-25/-13/63/66 -CSCM/oleron -JZ3c -p30/30 -Qs -N -png map

Actual output	Expected output

The error is because we forgot to set Ctrl->N.active to true if setting -N alone without level. This PR fixes the issue.