Fix some libcudf functions to set the null count on returning columns

[davidwendt]

Description

Fixes a few more places where the null count was not being set in a libcudf function before returning a column. A couple of places were setting the null count value into the mutable-view object. So the value was calculated correctly but not set into the actual column object.

Reference: #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.

[davidwendt]

/merge