Gisaid assembly files produced for empty consensus fastas

[michellescribner]

If a sample with very low coverage results in an empty consensus assembly file, the number_N column created by the theiacov workflows will show 0 Ns. The Mercury Prep workflow then produces gisaid assembly files for that sample because they have less Ns than the maximum allowed value. However, the Mercury Batch workflow will exclude the sample from the concatenated assembly.