RUNW/GUNW `ionospherePhaseScreen` raster values if ionosphere is turned off?

This Issue is re: RUNW/GUNW ionospherePhaseScreen.

Three cases:

1. There is high coherence, etc. and the ionosphere is ok
   • Great! The InSAR workflow generates the ionospherePhaseScreen raster normally.
2. The coherence is very low and the interferogram is bad.
   • Science team requested that the ionospherePhaseScreen raster be set to all NaN values. They did not want these set to all zeros (0s), because 0 has a meaning.
   • This change was introduced into the InSAR workflow in PR ???. (@vbrancat ? @oberonia78 ?)
   • QA was updated to not fail if ionospherePhaseScreen is all-NaN in: https://github.com/isce-framework/nisarqa/pull/74
3. The ionosphere is turned off in the InSAR runconfig
   • In this case, the InSAR workflow initialized the ionospherePhaseScreen raster to all-zeros, and then those values are never updated later in the workflow.

Question: For Case 3 when the ionosphere is turned off, should the raster be updated to be all-NaN values (same as in Case 2)?

If the value 0 has actual meaning, then I think it would be misleading to set the raster to all-zeros in Case 3?

Note: If it is decided to update Case 3 so that the raster is all-NaN when the ionosphere is turned off, then QA should consider updates to no longer allow an all-Zero ionospherePhaseScreen to be ok. Please keep QA informed of any updates, thank you!