TST: eliminate warning about unary_union

[martinfleis]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.9%. Comparing base (95829c9) to head (46d9287).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #913     +/-   ##
=======================================
+ Coverage   85.6%   85.9%   +0.2%     
=======================================
  Files        151     151             
  Lines      16310   16461    +151     
=======================================
+ Hits       13967   14134    +167     
+ Misses      2343    2327     -16     

Files with missing lines	Coverage Δ
libpysal/graph/tests/test_base.py	100.0% <ø> (ø)

... and 2 files with indirect coverage changes

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[jGaboardi]

• failures unrelated to this PR
• see #907