Problems after upgrading to 2.6.1

[pietrolesci]

Describe the bug

Loading a dataset_dict from disk with load_from_disk is now creating a KeyError "length" that was not occurring in v2.5.2.

Context:

• Each individual dataset in the dict is created with Dataset.from_pandas
• The dataset_dict is create from a dict of Datasets, e.g., `DatasetDict({"train": train_ds, "validation": val_ds})
• The pandas dataframe, besides text columns, has a column with a dictionary inside and potentially different keys in each row. Correctly the Dataset.from_pandas function adds key: None to all dictionaries in each row so that the schema can be correctly inferred.

Steps to reproduce the bug

Steps to reproduce:

• Upgrade to datasets==2.6.1
• Create a dataset from pandas dataframe with Dataset.from_pandas
• Create a dataset_dict from a dict of Datasets, e.g., `DatasetDict({"train": train_ds, "validation": val_ds})
• Save to disk with the save function

Expected behavior

Same as in v2.5.2, that is load from disk without errors

Environment info

• datasets version: 2.6.1
• Platform: Linux-5.4.209-129.367.amzn2int.x86_64-x86_64-with-glibc2.26
• Python version: 3.9.13
• PyArrow version: 9.0.0
• Pandas version: 1.5.1