MutableArrayData::extend_nulls doesn't update null bitmask

[tustvold]

Describe the bug

Noticed whilst working on #1225, calling MutableArrayData::extend_nulls doesn't update the null bitmask. This means the final array may have a shifted or completely invalid bitmask.

#1225 changed it so that if MutableArrayData is created with nulls disabled, it panics if you try to append nulls. Unfortunately it would appear this is being exploited by ArrowArrayReader to avoid computing a bitmask.

To Reproduce

#[test]
    fn test_nulls() {
        let ints: ArrayRef = Arc::new(Int32Array::from(vec![1]));
        let mut data = MutableArrayData::new(vec![ints.data()], true, 3);
        data.extend_nulls(9);
        let data = data.freeze();
        
        assert_eq!(data.len(), 9);
        assert_eq!(data.null_buffer().unwrap().len(), 2);
    }

Expected behavior

Appending nulls should update the null bitmask, and it should panic if MutableArrayData is created without null support. Fixing this is likely blocked on #1197 as it relies on this behaviour

[HaoYang670]

Hi, I am curious about what is index used to do here:

    /// Extends this [MutableArrayData] with elements from the bounded [ArrayData] at `start`
    /// and for a size of `len`.
    /// # Panic
    /// This function panics if the range is out of bounds, i.e. if `start + len >= array.len()`.
    pub fn extend(&mut self, index: usize, start: usize, end: usize) {
        let len = end - start;
        (self.extend_null_bits[index])(&mut self.data, start, len);
        (self.extend_values[index])(&mut self.data, index, start, len);
        self.data.len += len;
    }

[tustvold]

It's for when MutableArrayData is created from multiple source arrays, and is used to select which one to copy values from. The concat kernel makes use of this functionality at the moment