CAS-13318 : Fix PB masking in robust RMS calc
The initial bug seems to be because the outlierfield lies outside the
specified PB mask, and ends up being all masked pixels. This causes the
RMS estimation to fail, as well as deconvolution.
The fix is to switch the default mask to be unmasked in an outlierfield,
and after the PB mask is applied if the image is completely masked,
temporarily unmask it for RMS estimation.
This fixes the initial issue reported in the ticket, but investigating
if there are any unintended effects.
