Fix embedding function assignment in QdrantVectorDB

[eddyfidel]

Why are these changes needed?

These changes are needed to ensure that the QdrantVectorDB class behaves correctly when a custom embedding_function is provided. Previously, even if a custom function was passed, the code always defaulted to FastEmbedEmbeddingFunction(), which could lead to unexpected behavior and limit the flexibility of the class. By fixing this, we ensure that the custom embedding function is properly utilized, and only default to FastEmbedEmbeddingFunction() when no custom function is specified. This enhances the reliability and correctness of the embedding process within the class.

Related issue number

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• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[thinkall]

Nice catch! Thank you for the PR, @efidel .