Try to find places to deduplicate code in vtable implementations

[connortsui20]

I feel like this would be a good helper function for all of our arrays to make use of. I bet there is a lot of duplication in our vtable impls in general.

Originally posted by @connortsui20 in #8803

[connortsui20]

/// The validity expression for a binary geo kernel: the result is null iff either operand is
/// null. Returning this lets the planner derive the output's null mask symbolically (mask
/// pushdown) instead of executing the whole kernel.
pub(crate) fn binary_result_validity(expression: &Expression) -> VortexResult<Option<Expression>> {
    Ok(Some(and(
        expression.child(0).validity()?,
        expression.child(1).validity()?,
    )))
}

[connortsui20]

@HarukiMoriarty I think one of your PRs closed this?