Fix to handle missing multiTenancyConfig key

[edugonza]

fixes #884

[dirkkul]

HI @edugonza ,

Thanks a lot for your contribution! Would you be able to add a test for this?

You'd also need to agree to our CLA

[edugonza]

Hi @dirkkul,

I just added a mock test. I agree with the CLA.

[weaviate-git-bot]

To avoid any confusion in the future about your contribution to Weaviate, we work with a Contributor License Agreement. If you agree, you can simply add a comment to this PR that you agree with the CLA so that we can merge.

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