Fix: Always pass bytes in the RDKitMolCodec

[cwognum]

Changelogs

• Always pass bytes to the VLenBytes codec, parent class of the RDKitMolCodec.

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Checklist:

• Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
• Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
• Update the API documentation if a new function is added, or an existing one is deleted.
• Write concise and explanatory changelogs above.
• If possible, assign one of the following labels to the PR: feature, fix, chore, documentation or test (or ask a maintainer to do it for you).

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I was running into a Cython issue with a chunk size > 1 because we were passing None rather than bytes.

[jstlaurent]

👍