Fields within a null struct are not initialized with null values

### Describe the bug, including details regarding any error messages, version, and platform.

When creating an array from a python dict, field entries of a null struct are initialized with default values rather than null even if their field is nullable. In the minimal example below, you would expect the 3rd row to have values of `inner_1` and `inner_2` to be null.

```
import pyarrow as pa

print(pa.array([
{"outer": {"inner_1": 1, "inner_2": 2}},
{"outer": {"inner_1": 3, "inner_2": None}},
{"outer": None},
]))
```

Generates the following output:

```
-- is_valid: all not null
-- child 0 type: struct
-- is_valid:
[
true,
true,
false
]
-- child 0 type: int64
[
1,
3,
0
]
-- child 1 type: int64
[
2,
null,
0
]
```

### Component(s)

Python