fix handling of uninitialized vlen strings

[kmuehlbauer]

Description

Reading (partly) uninitialized vlen string datasets breaks because NULL values are not properly handled when fetching from global heap. Also fillvalue's need proper distribution.

Closes #109

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Checklist

• This pull request has a descriptive title and labels
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• Unit tests have been added (if codecov test fails)
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[kmuehlbauer]

OK, first I've changed the test to have uninitialized values to really see it break. Then, I've added the fix (and some more fixes 😬).

Getting this in would help for the h5netcdf integration. I somehow deactivated vlen datasets over there and found it breaking with uninitialized data after reinstating the tests proper.

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.31%. Comparing base (4512046) to head (20f70b6).
⚠️ Report is 182 commits behind head on main.

Files with missing lines	Patch %	Lines
pyfive/misc_low_level.py	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
+ Coverage   72.73%   73.31%   +0.57%     
==========================================
  Files          11       11              
  Lines        2505     2548      +43     
  Branches      382      394      +12     
==========================================
+ Hits         1822     1868      +46     
+ Misses        580      576       -4     
- Partials      103      104       +1     

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[kmuehlbauer]

I've added a similar fix for vlen attributes (found via h5netcdf testsuite).

[valeriupredoi]

@kmuehlbauer cheers - just let me know when this one's good to go in, mate 🍺
Also if @bnlawrence has a minute to look at it, that'd be fab

[kmuehlbauer]

just let me know when this one's good to go in, mate 🍺

I'll add a test for the uninitialized attribute for completeness. Hang on some minutes 😁

[kmuehlbauer]

I've had to recreate the breaking test from the h5netcdf testsuite. It used some mismatched dimension scales to trigger the error. I've put it in the test_references.py.

[kmuehlbauer]

Think this is ready from my side, when the lights get green. 💚