length on slices with null doesn't work

[bjchambers]

Describe the bug

length kernel (for string length) doesn't work on sliced string arrays containing null.

To Reproduce

Modify the existing tests to include null:

    /// Tests with an offset
    #[test]
    fn bit_length_offsets() -> Result<()> {
        let a = StringArray::from(vec![Some("hello"), Some(" "), Some("world"), None]);
        let b = a.slice(1, 3);
        let result = bit_length(b.as_ref())?;
        let result: &Int32Array = as_primitive_array(&result);

        let expected = Int32Array::from(vec![Some(8), Some(40), None]);
        assert_eq!(&expected, result);

        Ok(())
    }

Expected behavior
Test passes.

Actual behavior
Test does not pass.

[matthewmturner]

@bjchambers i can try looking into this.

[bjchambers]

@matthewmturner I think it should be addressed by #745.

[matthewmturner]

@bjchambers ah missed the linked pull. thx.

[matthewmturner]

@bjchambers out of curiousity - do you know why the length_offsets test uses bit_length function instead of length?

[bjchambers]

Hmm... probably a copy-paste error when updating/adding the tests. Could you comment on the PR and I'll update/resolve when I get a chance?