nested sequence feature won't encode example if the first item of the outside sequence is an empty list

[function2-llx]

Describe the bug

As the title, nested sequence feature won't encode example if the first item of the outside sequence is an empty list.

Steps to reproduce the bug

from datasets import Features, Sequence, ClassLabel
features = Features({
    'x': Sequence(Sequence(ClassLabel(names=['a', 'b']))),
})
print(features.encode_batch({
    'x': [
        [['a'], ['b']],
        [[], ['b']],
    ]
}))

Expected results

print {'x': [[[0], [1]], [[], ['1']]]}

Actual results

print {'x': [[[0], [1]], [[], ['b']]]}

Environment info

• datasets version: 1.15.1
• Platform: Linux-5.13.0-21-generic-x86_64-with-glibc2.34
• Python version: 3.9.7
• PyArrow version: 6.0.0

Additional information

I think the issue stems from here.

[function2-llx]

knock knock

[mariosasko]

Hi, thanks for reporting! I've linked a PR that should fix the issue.

[function2-llx]

I've checked the PR and it looks great, thanks a lot!