[C++][Gandiva] Fix size 0 case in arena malloc

The size 0 case in arena malloc is causing sporadic errors due to null pointer being returned to functions like convert_from which treat this as an error.

**Reporter**: [Siddhant Rao](https://issues.apache.org/jira/browse/ARROW-16191)
#### PRs and other links:
- [GitHub Pull Request #12879](https://github.com/apache/arrow/pull/12879)

_{**Note**: *This issue was originally created as [ARROW-16191](https://issues.apache.org/jira/browse/ARROW-16191). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by reviewing the Gandiva arena malloc path and the convert_from functions named in the issue, then inspect linked pull request #12879 for the work already underway. Done means the size-0 case no longer returns a null pointer treated as an error, with the relevant Gandiva tests passing.

Written by the indexing model from the issue text.