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ARROW-16651 : [Python] Casting Table to new schema ignores nullability of fields (#14048)

```python
table = pa.table({'a': [None, 1], 'b': [None, True]})
new_schema = pa.schema([pa.field("a", "int64", nullable=True), pa.field("b", "bool", nullable=False)])
casted = table.cast(new_schema)

```

Now leads to
```
RuntimeError: Casting field 'b' with null values to non-nullable
```

Authored-by: kshitij12345 <kshitijkalambarkar@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>