fix(indexing): treat an empty list as an empty integer selection

[XXXXXQ-0206]

arr[[]] raises IndexError: unsupported selection item for basic indexing, while the same selection as an empty integer array works and NumPy accepts it:

a = zarr.create_array({}, data=np.arange(16).reshape(4, 4), chunks=(2, 2))
a[[]]                            # IndexError
a[np.array([], dtype=np.intp)]   # (0, 4)
np.arange(16).reshape(4, 4)[[]]  # (0, 4)

Why it is wrong:

1. IntSequence (declared at src/zarr/core/indexing.py:42) includes list[int], so an empty list is a valid selection.
2. The empty array form already works inside the library (is_integer_array ignores length) and returns (0, 4); only the list form is rejected.
3. NumPy treats [] as an empty integer selection.

Cause: is_integer_list (:233) and is_bool_list (:238) both require len(x) > 0, so an empty list falls through to BasicIndexer, which accepts only int and slice.

The change drops len(x) > 0 from is_integer_list only and makes replace_lists use np.intp for an empty list (it otherwise infers float64). is_bool_list keeps its guard, so an empty list is an empty integer selection, not an empty mask, as in NumPy.

The new test covers empty lists in bare, tuple, oindex and vindex forms, plus the empty-array, boolean-list and write controls. It fails on the base revision (src blob 8ea59eb9 -> 2926551a); tests/test_indexing.py 592 -> 594 passed. Repo-wide pytest: 12527 passed, 1293 skipped, 4 xfailed (Windows, CPython 3.14.7, 24m); ruff check/format --check clean.

If you would rather keep an empty list raising, we are happy to adjust.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.71%. Comparing base (bd0dc84) to head (202518f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4518   +/-   ##
=======================================
  Coverage   94.71%   94.71%           
=======================================
  Files          94       94           
  Lines       13663    13665    +2     
=======================================
+ Hits        12941    12943    +2     
  Misses        722      722           

Files with missing lines	Coverage Δ
src/zarr/core/indexing.py	96.58% <100.00%> (+<0.01%)	⬆️

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