[cpp] fix duckdb benchmark_runner bugs

[whzruc]

python run_benchmark_simple.py --dir=benchmark/clickbench/clickbench-pixels-e0-1ssd/

[whzruc]

The reason for this bug is that the implementation of the String type in pixels-cpp is incorrect. I also tested other types. The results are shown as follows.

• string(varchar)
  
• Select *
  
• BufferPool?

I think the work is not easy,may need to rewrite some of the pixels-cpp