Fix NestedExtensionArray.flat_length

[hombit]

This failed for empty chunks, because chunk.field(0).value_lengths().sum().as_py() returned None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.41%. Comparing base (6798700) to head (856d7bf).
Report is 184 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #196   +/-   ##
=======================================
  Coverage   98.41%   98.41%           
=======================================
  Files          13       13           
  Lines        1138     1138           
=======================================
  Hits         1120     1120           
  Misses         18       18           

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[github-actions]

Before [6798700] <v0.3.5>	After [049235d]	Ratio	Benchmark (Parameter)
25.0±0.8ms	25.9±0.2ms	1.04	benchmarks.AssignSingleDfToNestedSeries.time_run
32.8±1ms	33.0±8ms	1.01	benchmarks.ReassignHalfOfNestedSeries.time_run
270M	270M	1	benchmarks.AssignSingleDfToNestedSeries.peakmem_run
92M	92.3M	1	benchmarks.NestedFrameAddNested.peakmem_run
9.58±0.07ms	9.58±0.08ms	1	benchmarks.NestedFrameAddNested.time_run
96.4M	96.4M	1	benchmarks.NestedFrameQuery.peakmem_run
10.6±0.08ms	10.6±0.08ms	1	benchmarks.NestedFrameQuery.time_run
95.7M	95.6M	1	benchmarks.NestedFrameReduce.peakmem_run
1.27±0.01ms	1.27±0.04ms	1	benchmarks.NestedFrameReduce.time_run
289M	289M	1	benchmarks.ReassignHalfOfNestedSeries.peakmem_run

Click here to view all benchmarks.

[gitosaurus]

LGTM!