LabelEncoder doesn't handle missing values in *dask* series of strings

**Describe the issue**:

When using a LabelEncoder on a dask series with missing values (as `np.nan`), a TypeError is raised with "<" being undefined for floats and strings.

scikit-learn's encoder seems to handle this well for pandas and dask series. We seem to handle it well with a pandas series.

**Minimal Complete Verifiable Example**:

```python
import dask.dataframe as dd
from dask_ml.preprocessing import LabelEncoder as dask_le
from sklearn.preprocessing import LabelEncoder as skl_le
import numpy as np
import pandas as pd

df = pd.DataFrame({
"A": list("aaaabbbcccdddeeefffgggg")
})

df.loc[[0, 2, 5, 10, 21], "A"] = np.nan

ddf = dd.from_pandas(df, npartitions=3)

# works
lenc = skl_le().fit(df["A"])
lenc = skl_le().fit(ddf["A"])
lenc = dask_le().fit(df["A"])

# fails
lenc = dask_le().fit(ddf["A"])

# but also works
lenc = dask_le().fit(ddf["A"].fillna(""))
```

**Full Trackback**:

➜ python label_encoder_repro.py

Traceback (most recent call last):

  File "/Users/paul/work/sources/dask-engineering/example-pipelines/criteo-HPO/label_encoder_repro.py", line 21, in 

    lenc = dask_le().fit(ddf["A"])

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask_ml/preprocessing/label.py", line 119, in fit

    self.classes_ = classes_.compute()

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/base.py", line 315, in compute

    (result,) = compute(self, traverse=False, **kwargs)

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/base.py", line 600, in compute

    results = schedule(dsk, keys, **kwargs)

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/threaded.py", line 89, in get

    results = get_async(

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/local.py", line 511, in get_async

    raise_exception(exc, tb)

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/local.py", line 319, in reraise

    raise exc

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/local.py", line 224, in execute_task

    result = _execute_task(task, data)

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/core.py", line 119, in _execute_task

    return func(*(_execute_task(a, cache) for a in args))

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/optimization.py", line 990, in __call__

    return core.get(self.dsk, self.outkey, dict(zip(self.inkeys, args)))

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/core.py", line 149, in get

    result = _execute_task(task, cache)

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/core.py", line 119, in _execute_task

    return func(*(_execute_task(a, cache) for a in args))

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/core.py", line 119, in 

    return func(*(_execute_task(a, cache) for a in args))

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/core.py", line 119, in _execute_task

    return func(*(_execute_task(a, cache) for a in args))

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/utils.py", line 71, in apply

    return func(*args, **kwargs)

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/dask/array/routines.py", line 1626, in _unique_internal

    u = np.unique(ar)

  File "<__array_function__ internals>", line 180, in unique

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/numpy/lib/arraysetops.py", line 274, in unique

    ret = _unique1d(ar, return_index, return_inverse, return_counts, 

  File "/Users/paul/mambaforge/envs/ml-example/lib/python3.10/site-packages/numpy/lib/arraysetops.py", line 336, in _unique1d

    ar.sort()

TypeError: '<' not supported between instances of 'str' and 'float'

**Environment**:

- Dask version: 2012.12.0
- Python version: 3.10
- Operating System: M1 Mac
- Install method (conda, pip, source): conda