bug(steps): handle col with all nulls in impute

[jitingxu1]

For all the impute, we need to handle cols with all nulls. We need to tell the user the column is all nulls by failing the imputing or throw warnings.

I prefer fail the impute.

import ibis
import ibis_ml as ml
import numpy as np

a = ibis.memtable(
    {"all_null": np.array([np.nan, np.nan, np.nan], dtype="float64")}
)
step = ml.ImputeMean(ml.numeric())
step.fit_table(a, ml.core.Metadata())
step.transform_table(a)

┏━━━━━━━━━━┓
┃ all_null ┃
┡━━━━━━━━━━┩
│ float64  │
├──────────┤
│      nan │
│      nan │
│      nan │
└──────────┘