BUG: fix a case when there's only a single building to be passed to voronoi_frames

[u3ks]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.0%. Comparing base (4037c70) to head (b7b11fd).
Report is 56 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #621     +/-   ##
=======================================
+ Coverage   97.4%   98.0%   +0.6%     
=======================================
  Files         26      38     +12     
  Lines       4328    6176   +1848     
=======================================
+ Hits        4214    6051   +1837     
- Misses       114     125     +11     

Files	Coverage Δ
momepy/functional/_elements.py	99.1% <100.0%> (ø)
momepy/functional/tests/test_elements.py	99.0% <100.0%> (ø)