fix: "None" in key path when no prefix is specified

[ReubenFrankel]

Previously, if you didn't specify prefix config you would end up with a file path containing None

2024-09-13 12:19:31,494 | INFO     | target-s3            | key: my-bucket/None/my-stream/.jsonl.gz

or duplicate forward slashes if set to an empty string

2024-09-13 12:19:31,494 | INFO     | target-s3            | key: my-bucket//my-stream/.jsonl.gz

both of which result in an undesirable structure in S3:

This change handles prefix optionally for the key, using pathlib to normalise the path.

2024-09-13 12:42:10,206 | INFO     | target-s3            | key: my-bucket/my-stream.jsonl.gz

A similar issue was resolved where the file name stem in the generated key path ended with a slash (as above) when disabling some of the append_ settings:

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Fixes #19

[ReubenFrankel]

Related to #38