test_card fails when preprocess_steps is None

[marukaz]

After I updated unitxt from 1.7.6 to 1.7.7, if there is no preprocess_steps, test_card() fails with

  File "/Users/marukaz/ghq/github.com/marukaz/unitxt/src/unitxt/standard.py", line 293, in prepare
    super().prepare()
  File "/Users/marukaz/ghq/github.com/marukaz/unitxt/src/unitxt/standard.py", line 215, in prepare
    self.standardization.steps.extend(self.card.preprocess_steps)
TypeError: 'NoneType' object is not iterable

Example card:

from unitxt.blocks import (
    AddFields,
    LoadHF,
    TaskCard,
)
from unitxt.catalog import add_to_catalog
from unitxt.test_utils.card import test_card

card = TaskCard(
    loader=LoadHF(path="EdinburghNLP/xsum"),
    # preprocess_steps=[
    #     AddFields(fields={"document_type": "document"}),
    # ],
    task="tasks.summarization.abstractive",
    templates="templates.summarization.abstractive.all",
)

test_card(card)
add_to_catalog(card, "cards.xsum", overwrite=True)

The cause is None check has been removed in this PR https://github.com/IBM/unitxt/pull/739/files#diff-a4122a708bc5f51b81dc0316fb8034bff822104b0e9d82bbe30d44761979ec36L144

I think either the code for the None check should be reinstated, or the default for preprocess_steps should be changed from None to an empty list.