Fix loading compressed CSV without streaming
When implementing support to stream CSV files (https://github.com/huggingface/datasets/commit/ad489d4597381fc2d12c77841642cbeaecf7a2e0#diff-6f60f8d0552b75be8b3bfd09994480fd60dcd4e7eb08d02f721218c3acdd2782), a regression was introduced preventing loading compressed CSV files in non-streaming mode.

This PR fixes it, allowing loading compressed/uncompressed CSV files in streaming/non-streaming mode.

Fix #2977.