Divide by zero RuntimeWarnings for archive downloads

[talonmyburgh]

When downloading from archive using the mvftoms.py tool like so:
mvftoms.py --applycal l1 --flags cam,data_lost -f --target J2236-2224 --target-tag target --chanbin 32 <rdb-token>

I note RuntimeWarnings:

<path>/site-packages/katdal/ms_extra.py:392: RuntimeWarning: divide by zero encountered in power
  main_dict['SIGMA_SPECTRUM'] = weight_data ** -0.5
<path>/site-packages/katdal/ms_extra.py:406: RuntimeWarning: divide by zero encountered in power
  main_dict['SIGMA'] = np.mean(weight_data, axis=1) ** -0.5

where <path> is my local venv path.

I've found this to occur for commit 0840fd8 and prior for an assortment of datasets and differing arguments.

The dataset appears fine after download but I've not done a very thorough inspection.