Support JSON lines with missing struct fields

[albertvillanova]

Support JSON lines with missing struct fields.

Fix #7159.

The implemented test raised:

TypeError: Couldn't cast array of type
struct<age: int64>
to
{'age': Value(dtype='int32', id=None), 'name': Value(dtype='string', id=None)}

[HuggingFaceDocBuilderDev]

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