Fix #242: correctly reset varchar chunk when casting data in CopyChunk #167

Job	Run time
Build extension binaries / Generate matrix	22s
Build extension binaries / Windows (windows_amd64, x64-windows-static-md)	32m 17s
Build extension binaries / Linux (linux_amd64, ubuntu:18.04, x64-linux)	4m 47s
Build extension binaries / DuckDB-Wasm	0s
Build extension binaries / Linux (linux_amd64_gcc4, quay.io/pypa/manylinux2014_x86_64, x64-linux)	4m 49s
Build extension binaries / MacOS (osx_amd64, x86_64, x64-osx)	4m 1s
Build extension binaries / Linux (linux_arm64, ubuntu:18.04, arm64-linux)	5m 4s
Build extension binaries / MacOS (osx_arm64, arm64, arm64-osx)	3m 5s
Deploy extension binaries / Generate matrix	23s
Deploy extension binaries / Deploy (linux_amd64)	31s
Deploy extension binaries / Deploy (linux_amd64_gcc4)	31s
Deploy extension binaries / Deploy (linux_arm64)	30s
Deploy extension binaries / Deploy (osx_amd64)	30s
Deploy extension binaries / Deploy (osx_arm64)	29s
Deploy extension binaries / Deploy (windows_amd64)	38s
	57m 57s

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