Fix error when calling to_xarray_dataset on an empty scene

[BENR0]

If to_xarray_dataset is called on an empty Scene instead of an error an empty xarray dataset is now returned.

• Closes to_xarray_dataset on empty scene fails with TypeError #1584
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #1698 (23168a6) into main (25993ff) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1698      +/-   ##
==========================================
+ Coverage   92.32%   92.42%   +0.10%     
==========================================
  Files         258      258              
  Lines       38125    37625     -500     
==========================================
- Hits        35200    34776     -424     
+ Misses       2925     2849      -76     

Flag	Coverage Δ
behaviourtests	4.65% <0.00%> (-0.19%)	⬇️
unittests	92.98% <100.00%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/scene.py	92.22% <100.00%> (-0.07%)	⬇️
satpy/tests/test_scene.py	99.44% <100.00%> (+<0.01%)	⬆️
satpy/tests/features/steps/steps-load.py	49.12% <0.00%> (-6.94%)	⬇️
...ts/features/steps/steps-real-load-process-write.py	30.76% <0.00%> (-4.95%)	⬇️
satpy/readers/maia.py	19.27% <0.00%> (-1.90%)	⬇️
satpy/readers/abi_base.py	93.00% <0.00%> (-1.59%)	⬇️
satpy/readers/goes_imager_nc.py	63.88% <0.00%> (-1.59%)	⬇️
satpy/readers/__init__.py	95.51% <0.00%> (-1.16%)	⬇️
satpy/readers/olci_nc.py	90.29% <0.00%> (-1.14%)	⬇️
satpy/readers/viirs_edr_active_fires.py	89.28% <0.00%> (-1.04%)	⬇️
... and 151 more

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[coveralls]

Coverage increased (+0.2%) to 93.041% when pulling 23168a6 on BENR0:fix_to_xarray_dataset_empty_scene into 25993ff on pytroll:main.

[gerritholl]

This needs a unit test. It should be quite easy to write a unit test for this case.

[djhoese]

@BENR0 Can you add a small test for this?

[BENR0]

@djhoese Added a test for the empty Scene.

[djhoese]

Thanks!