fix: allow Nullable targetElement

[magic-akari]

Summary

What kind of change does this PR introduce? (check at least one)

• [ ] Bug fix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Docs
• [ ] Build-related changes
• [x] Other, please describe: Types

The PR fulfills these requirements:

• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following platforms:

• [ ] PC
• [ ] iOS
• [ ] Android

Providing a reproduction demo will be better(you can pick a platform below and fork the demo):

• jsbin: https://jsbin.com/cafiful/edit?output
• codepen: https://codepen.io/buptsteve/pen/PvNQjP
• jsfiddle: https://jsfiddle.net/buptsteve/6u8g3Lf5/
• codesandbox: https://codesandbox.io/s/o73z4jy5q9

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

Why Nullable:

document.querySelector() or document.getElementById() return Nullable result.

Writing lock(document.querySelector('.target')) is more concise than writing lock(document.querySelector('.target') || undefined)

[BuptStEve]

LGTM

[BuptStEve]

@all-contributors add @magic-akari for bug

[allcontributors[bot]]

@BuptStEve

I've put up a pull request to add @magic-akari! :tada: