Update Fuzz tests to include Dict with null values

#16258 closes #16228 

We should [update the fuzz tests to include Dict with null values](https://github.com/apache/datafusion/pull/16232/files#diff-08d7a1f4d6a968c393a2a0f2a2f54118f38d6a29009ce31b261f3ca27a2d3396R733).

source - [comment on 16258](https://github.com/apache/datafusion/pull/16258#issuecomment-2944973123)