Function parse_expr 

fn parse_expr(
    input_exprs: &[Arc<dyn PhysicalExpr>],
    input_fields: &[FieldRef],
) -> Result<Arc<dyn PhysicalExpr>>

When lead/lag is evaluated on a NULL expression we attempt to refine it by matching it with the type of the default value.

For e.g. in lead(NULL, 1, false) the generic ScalarValue::Null is refined into ScalarValue::Boolean(None). Only the type is refined, the expression value remains NULL.

When the window function is evaluated with NULL expression this guarantees that the type matches with that of the default value.

For more details see: https://github.com/apache/datafusion/issues/12717