fix: left join with no matching rows crashed queries on labels

[FarisHrvat]

Querying a labels element that is annotated by a table, but whose cropped area contains none of the annotated instances, crashes polygon_query and bounding_box_query with IndexError: list index out of range in _region_as_str_if_list_of_len_one. The left join builds a 0-row table and then reads the first element of an empty region list.
This now returns no table in that case, like the inner and exclusive joins already do, so the query keeps the labels and drops the table.
It's also the random failure of test_attributes_are_copied[labels2d-*] in the pre-release job on #1265. The labels there come from the shared RNG in conftest.py, so whether the small query box contains an annotated instance depends on which tests ran before on the same worker. Added a test that builds that case on purpose.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.40%. Comparing base (ea93a37) to head (5377b7f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1266   +/-   ##
=======================================
  Coverage   91.40%   91.40%           
=======================================
  Files          53       53           
  Lines        8381     8381           
=======================================
  Hits         7661     7661           
  Misses        720      720           

Files with missing lines	Coverage Δ
src/spatialdata/_core/query/relational_query.py	93.34% <100.00%> (ø)

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