Remove null mask and null count from column_view constructors

[vyasr]

Description

This is a breaking change that removes default values. Removing the default null count value of UNKNOWN_NULL_COUNT is necessary since we are removing UNKNOWN_NULL_COUNT altogether. A potential alternative was setting the default to 0, which would correspond to a default null mask of nullptr (the current default). However, that change is potentially more dangerous if callers were previously setting the null mask to something other than nullptr without setting the null count and relying on the behavior of UNKNOWN_NULL_COUNT. Removing the default parameter altogether is therefore the safer option here.

Contributes to #11968.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[GregoryKimball]

@etseidl I'd like to make sure you are aware of this work. Would you please let me know if you are using mutable_column_view in your application? If so, I recommend checking the null mask and null count handling to make sure the data structures stay consistent.

[vyasr]

To be clear, @etseidl this particular PR won't affect anything major regarding the mutable column view. The problematic case will be in a subsequent PR (related to this overall project). The relevant change will affect the following current behavior:

1. Create and populate a column
2. Create a mutable view of that column
3. Modify the null mask via the mutable view in place

Currently, the creation of the mutable view (step 2) unsets the null count in the column, so the next time that the column's null count is queried it is recomputed based on the mask. In the future, that lazy computation of the count will be removed, so the user will be required to set the null count as part of step 3 (setting the mask alone will be insufficient).

[vyasr]

/merge