[Python][C++] A file with one line of JSON without a line ending is incorrectly loaded when used as a Dataset

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

### Description
If a file has one line of JSON, and there is no line ending at the end of the file, `pyarrow.dataset.dataset` will correctly infer a schema, but then fail to load the values from that row.

### Reproduction
Bug case:
```shell
echo -n '{"field": 1}' > test.json
python3 -c 'import pyarrow.dataset; print(pyarrow.dataset.dataset("test.json", format="json").to_table().to_pandas())'
field
0 NaN
```

With a newline it works as I'd expect:
```shell
echo '{"field": 1}' > test.json
python3 -c 'import pyarrow.dataset; print(pyarrow.dataset.dataset("test.json", format="json").to_table().to_pandas())'
field
0 1
```

With multiple lines but no final trailing newline it _also_ works as I'd expect:
```shell
echo -en '{"field": 1}\n{"field": 2}' > test.json
python3 -c 'import pyarrow.dataset; print(pyarrow.dataset.dataset("test.json", format="json").to_table().to_pandas())'
field
0 1
1 2
```

You'll notice it's inferring the schema correctly in all cases (it knows there's one field named `field`). If I change the data type to string, it also changes the default null value correctly, even though the value shouldn't be null:
```shell
echo -n '{"field": "value"}' > test.json
python3 -c 'import pyarrow.dataset; print(pyarrow.dataset.dataset("test.json", format="json").to_table().to_pandas())'
field
0 None
```

AFAICT, this only affects datasets. `pyarrow.json.read_json()` works just fine, for example:
```shell
echo -n '{"field": 1}' > test.json
python3 -c 'import pyarrow.json; print(pyarrow.json.read_json("test.json").to_pandas())'
field
0 1
```

### Debug info
Python: 3.12.3
PyArrow: 19.0.0
OS: Linux - Ubuntu 24.04.1

(I'm reporting this against the Python component because that's what easy for me to test locally and where I saw the issue, but I assume that the actual bug is lower in the stack.)

### Component(s)

Python