[pyarrow] fixed size binary arrays with nulls consume too much memory

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

Fixed size binary arrays with nulls use full fixed length size of memory for nulls.
This demonstrates the issue
```
len = 1000000 # 1 mio
nulls = 1000 # 1k nulls
x = [b'0'*len]+[None]*nulls # 1 value of 1M bytes, and 1000 nulls
arr = pa.array(x, pa.binary(len))
print(f"allocated: {pa.total_allocated_bytes()}") # should print about 1MB but prints 1001*1MB.
```

Same thing happens if I load parquet data with fixed size binary columns with many nulls. Actually it makes those files unreadable, as memory consumption is off the charts.

### Component(s)

Python