understand that null responses can be returned for the key/value array

[sjpotter]

in #2772 users noted that we are throwing a javascript exception as we are trying to apply .length to a null and hence that fails.

we have 2 options. First, we can just elide these results from the response we give the user, but this wouldn't correspond to the ft.search documentation. Therefore, instead, as redis doesn't return a document, but a null, we should do the same, and return an empty null object for these documents. It's up to the user to handle them

fixes #2772

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• Does npm test pass with this change (including linting)?
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