[FEA] Add null_count_if_available or lazy_null_count to column and column_view

[ttnghia]

The classes column and column_view have null_count functions that return the number of nulls in the given column. The column holds an internal null count value which is defaulted to UNKNOW_NULL_COUNT.

During various transformations, we need to copy that null count value around but may not really care about what that value is. However, if the function null_count() is called but the column's internal number of nulls has not been computed, a kernel will always be launched to compute the actual null count. That is a waste of time because we can just return UNKNOW_NULL_COUNT without compute anything.

I suggest adding a new function that allows returning the column's internal null count directly without computation. That function can be named null_count_if_availabe or lazy_null_count or anything else that is relevant.

[davidwendt]

The UNKNOWN_NULL_COUNT feature will be removed with #11968
So we will not be needing these extra functions.

[GregoryKimball]

I'll close this in favor of #11968 for now