fix(useMounted): The second argument in vue2.7 should be undefined

[yanbowe]

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⚠️ Slowing down new functions

Warning: Slowing down new functions

As the VueUse audience continues to grow, we have been inundated with an overwhelming number of feature requests and pull requests. As a result, maintaining the project has become increasingly challenging and has stretched our capacity to its limits. As such, in the near future, we may need to slow down our acceptance of new features and prioritize the stability and quality of existing functions. Please note that new features for VueUse may not be accepted at this time. If you have any new ideas, we suggest that you first incorporate them into your own codebase, iterate on them to suit your needs, and assess their generalizability. If you strongly believe that your ideas are beneficial to the community, you may submit a pull request along with your use cases, and we would be happy to review and discuss them. Thank you for your understanding.

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Description

#3803

Additional context

[Glandos]

Isn't it better to use undefined and let bundlers do their job by using void 0 if they want?

It's just my opinion, I prefer to read undefined, it's seems clearer to me.

[susnux]

Is there any plan for a new vueuse release with this?

[qnp]

Same, I'm looking forward to seeing this fix released, as I have errors that throw on my Vue2 project due to null target instead of undefined.