C++: Add cast from Dictionary[NullType] to NullType

[asfimport]

Cleanup after https://issues.apache.org/jira/browse/PARQUET-1121

Reporter: Uwe Korn / @xhochy
Assignee: Uwe Korn / @xhochy

PRs and other links:

• GitHub Pull Request #1189

_{Note: This issue was originally created as ARROW-1648. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 1189
#1189