v4: Fix useMutation "mutate" optional arguments

[bbugh]

This fixes a type error in useMutation's exported mutate function that required arguments to be specified, even though the intention was that they are optional.

(This appears to have been caused by a typo - the first argument of mutate had a default value, but the second one didn't. I'm surprised that TypeScript didn't catch this.)

const { mutate } = useMutation(...)

// ❌ v4-alpha4: Expected 2 arguments, but got 0
// ✅ works after this PR
mutate()

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This PR resolves one of several issues found while creating a graphql-code-generator plugin for vue-apollo hooks, which can be tracked here.