milvus: Avoid crashing when empty documents are supplied to AddDocuments

[AloizioMacedo]

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
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• Contains test coverage for new functions.
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Explanation

When interacting with different ways to create documents and then proceeding to AddDocuments, sometimes you can create an empty list of documents. This currently generates a crash due to line 211 of milvus.go. Other vectorstores (like chroma's) do not have this restriction, and there is no clear assumption that the documents have to be nonempty in the interface.

This PR makes AddDocuments a noop when empty documents are passed into it.