UniqueCombinations constraint on only one column gets stuck in an infinite loop

[fealho]

Using the UniqueCombinations constraint on only one column gets stuck in an infinite loop. This can be fixed by validating the constraint inputs, since the reason for the crash is that UniqueCombinations requires at least two column as input.

Code to reproduce the error:

import pandas as pd
from sdv.constraints import *
from sdv.tabular import *

zero_to_nine = [i for i in range(10)]
data = pd.DataFrame({
    'a': zero_to_nine,
    'b': zero_to_nine,
    'c': zero_to_nine
})

constraint = UniqueCombinations(
    columns=['a'],
)

model = CTGAN(constraints=[constraint])
model.fit(data)