Fetch is called in spite of conditional variables are undefined

[fkrafi]

Describe the bug
Fetch is called in spite of conditional variable value is undefined

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const {data} = useFetch(`/api/product/${id}`, {}, [id]);

Expected behavior
Fetch shouldn't call the api when the conditional variables are undefined

[iamthesiz]

1. you can just do useFetch(`api/product/${id}`, [id])
2. this is a feature that would need to be implemented. I'm open to it. Submit a PR. I think the best way to do this would be to check if the dependency is undefined.