Raise clear error for all-empty shapes

[timtreis]

Summary

• Detect shape elements that contain only empty geometries before calling get_extent in PlotAccessor.show.
• Replace the bare upstream AssertionError("Cannot compute extent of an empty collection of geometries.") with a ValueError that names the offending element(s) and coordinate system.

Closes #629.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.63%. Comparing base (59da170) to head (6715157).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/basic.py	50.00%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #674   +/-   ##
=======================================
  Coverage   77.63%   77.63%           
=======================================
  Files          11       11           
  Lines        3612     3630   +18     
  Branches      850      858    +8     
=======================================
+ Hits         2804     2818   +14     
- Misses        485      486    +1     
- Partials      323      326    +3     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/basic.py	86.12% <50.00%> (-0.48%)	⬇️

... and 2 files with indirect coverage changes

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