unique() panics on empty global categorical series

[mcrumiller]

Issue Description

Empty categorical array in a global context panics when unique is called.

import polars as pl

with pl.StringCache():
    pl.Series([], dtype=pl.Categorical).unique()
# pyo3_runtime.PanicException: attempt to subtract with overflow

Installed versions

main

[mcrumiller]

@ritchie46 this one panics because of this section: https://github.com/pola-rs/polars/blob/main/crates/polars-core/src/chunked_array/logical/categorical/ops/unique.rs#L40-L46

let mut min = u32::MAX;
let mut max = 0u32;

for &v in map.keys() {
    min = min.min(v);
    max = max.max(v);
}

PrimitiveRangedUniqueState::new(min, max + has_nulls)

If the series is empty then min remains as u32::MAX, I think we need a fast path for empty arrays.