fix: Add unique fast path for empty categoricals

[mcrumiller]

Closes #20528.

I don't think this will interfere with @coastalwhite's upcoming fixes.

Evidence of fix:

>>> import polars as pl
>>> pl.enable_string_cache()
>>> pl.Series(["aaa", "bbb", "ccc"])  # pre-fill cache
>>> s = pl.Series(["a", "b", "c"], dtype=pl.Categorical)
>>> s_empty = s.slice(0, 0)          # create empty array
>>> s_empty.unique()
shape: (0,)
Series: '' [cat]
[
]
>>> s_empty.cat.get_categories()
shape: (3,)
Series: '' [str]
[
        "a"
        "b"
        "c"
]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.04%. Comparing base (1ebd039) to head (b2c4196).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20536      +/-   ##
==========================================
- Coverage   79.04%   79.04%   -0.01%     
==========================================
  Files        1563     1564       +1     
  Lines      220798   220621     -177     
  Branches     2502     2502              
==========================================
- Hits       174522   174380     -142     
+ Misses      45702    45667      -35     
  Partials      574      574              

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