Nullable variable length vector fillEmpties() fills an extra value

[asfimport]

The fillEmpties() method is needed to update the offset vector in between non-null values. But it fills the current value which we are trying to set as well, when this is unnecessary. In fact, in the case of setValueCount(), this can result in an unnecessary reAlloc() call.

Reporter: Steven Phillips / @StevenMPhillips
Assignee: Steven Phillips / @StevenMPhillips

_{Note: This issue was originally created as ARROW-875. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 579
#579