In `assemble()`, we set the NO_DATA mask bit for any pixel where the variance == 0 through coaddUtils.setCoaddEdgeBits. This criteria was a shortcut to determine whether any warps contributed to a pixel's stacked value. It worked for the two statistics available at the time: MEAN and MEANCLIP. In
DM-4158 we now allow most available statistics, some of which (e.g. NPOINT, SUM) set the variance=0 intentionally, having the unfortunate consequence of rendering the whole mask with NO_DATA.
I'm marking this as dependent on
DM-10907, because it is trivial with a number map in hand.
With this change I'll also remove requirement in validate() that doInterp=False if a statistic with no uncertainties is used.