(RSLC) Missing `_FillValue` attribute on swath polarization datasets

Location: /science/LSAR/RSLC/swaths/frequency[A|B]/[HH|HV|VH|VV]

Description: XML schema expects the _FillValue attribute that is missing from all polarization datasets in the RSLC swaths.

From QA Log (example for one pol):

Attributes found in XML but not HDF5: {'_FillValue', 'isReceiveOnly'} - Dataset /science/LSAR/RSLC/swaths/frequencyA/HH

Impact: Schema non-compliance. Tools relying on this attribute to identify invalid/fill pixels may fail or produce incorrect results.

Notes:

• As noted in https://github.com/isce-framework/isce3/issues/297, per CF Conventions the _FillValue dtype should match exactly the dtype of the raster dataset it is attached to. (e.g. both complex64, both complex32, etc.)

If it helps, here is existing ISCE3 code which creates complex-valued fill value:

https://github.com/isce-framework/isce3/blob/67f08784a6768fca4ea5aeb6d4c77d2e731c8051/python/packages/nisar/workflows/h5_prep.py#L74-L87

Issue originally opened Apr 6, 2023: https://github-fn.jpl.nasa.gov/isce-3/isce/issues/1302

cc: @hfattahi @bhawkins

Start with the RSLC swath polarization datasets at /science/LSAR/RSLC/swaths/frequency[A|B]/[HH|HV|VH|VV], then compare their creation path with the complex-valued fill-value example in python/packages/nisar/workflows/h5_prep.py lines 74-87. Add the missing _FillValue attributes with dtypes matching their raster datasets, and verify that the XML and HDF5 attributes agree.