[pyarrow] Unable to write an array if the schema is nullable

[Rollo Konig-Brock]

Hey all,

I am unable to write an array (without any nulls) if the schema
explicitly sets nullable to false.

A quick example:

```
import io
import pyarrow

schema = pyarrow.schema([
    pyarrow.field('_strangle', 'string', nullable=False)]
)

batch_array = pyarrow.array(['abc', 'åbc', None])

output = io.BytesIO()

writer = pyarrow.RecordBatchStreamWriter(output, schema)

batch = pyarrow.RecordBatch.from_arrays(
    [batch_array], schema.names
)

# This fails with pyarrow.lib.ArrowInvalid: Tried to write record batch
with different schema
writer.write_batch(batch)
```

Any ideas here? It seems quite strange that I can't find an interface for
writing not-nullable arrays.

All the best,
Rollo