Fix sliced ConcatenationTable pickling with mixed schemas vertically
A sliced + pickled ConcatenationTable could end up with a different schema than the original schema, if the slice only contains blocks with only a subset of the columns.

This can lead to issues when saving datasets from a concatenation of datasets with mixed schemas

Reported in https://discuss.huggingface.co/t/datasetdict-save-to-disk-with-num-proc-1-seems-to-hang-with-error/75595